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So, I'm currently toying around with the idea of a hypercarnivorous sapient race on a planet in my universe, and I'd like to make them scary, but without abiding only by the "rule of cool". (Also note that the science, especially biology, involved in building this world is more important, to me, than the story)

A recent idea of mine regarding these sophonts is to have increased adrenaline (or an alien analogue of adrenaline) production dramatically speed up the transmission of signals inside the brain, and therefore (If I assume correctly), increase the creature's intelligence.

So, my question is: could an increase in adrenaline production cause a wholly noticeable change in the perceived "intelligence" of a species, and if so, how would this work? When I say "intelligence", I'm not really obeying any strict scientific definition; what I mean is that the creature would seem more intelligent in its behavior.

If I recall correctly, during intense moments (such as car crashes), and therefore times when you'd get an adrenaline rush, you take in much more details about the environment. Obviously, the sensory feedback would have to be processed, so perhaps something analogous to this does occur in reality; however, I'm looking for a noticeable difference, perhaps even as drastic as a chimpanzee suddenly thinking like a human.

I'm not (currently) asking if this kind of system could evolve naturally, but that's something I'll definitely be looking into in a future question.

You don't need adrenaline, you need heat radiators and more blood supply.

Adrenaline doesn't actually speed up the process of thinking, it acts as a switch to give your brain the fuel it needs to run faster.

From Your Hormones:

Key actions of adrenaline include increasing the heart rate,
increasing blood pressure, expanding the air passages of the lungs,
enlarging the pupil in the eye (see photo), redistributing blood to
the muscles and altering the body's metabolism, so as to maximize
blood glucose levels (primarily for the brain).

Our brains could actually run faster than they normally do but they would run out of oxygen and fuel and they would overheat.

So, if you can provide a steady amount of fuel and get rid of the waste heat faster, you can get the effect you want without adrenaline.

With a better heart, bigger lungs and a thicker neck (for more plumbing), you could have supply enough blood to give the high speed mode a longer run time. You could also pack an organ in or near the brain that stores oxygen and fuel (a sort of bio-capacitor). That capacitor would allow the creature to go high speed until it is empty. Then the capacitor could be recharged over time through blood flow.

For extra safety, have the bio-capacitor isolate the brain from the rest of the blood supply. This would protect the brain from diseases and toxins even better than our blood/brain barrier. It also means that even if you destroy its heart and lungs, it can still be killing things until the bio-capacitor runs out of oxygen and fuel.

The downside is that all of this will produce a lot of heat. Our brains produce a lot of heat. That's one of the reasons why it is in our head (a fairly vulnerable place) so it has more surface area for heat radiation. You will need radiators to get rid of that heat. If you go with the totally isolated brain, the radiators need to be in the head. If the brain can dump the heat into the blood stream, you can put the radiators on the body.

Adrenaline does help the brain function, but it is more about faster reactions (i.e. faster information processing), not about improving reasoning capacity.

Moreover, adrenaline triggers fight-or-flight mode in the brain, which is not exactly helpful for things like developing new tools, or planning hunting strategy, since they require calm and slow thinking. Still, adrenaline can help a creature find a crafty way to escape, or attack a vulnerable spot.

Moreover, adrenaline turbo-charges entire body, and continuous elevated level of adrenaline will wear it out. Kind of like how Nitrox makes a car go faster, but wears out the engine.

I would expect the exact opposite to occur, actually.

If you think about what the purpose of such a neurotransmitter would be, its job is to help you operate in environments where rapid decisions and adaptation is vital. This is the opposite of what we want for intelligent thinking, where we want time to mull over complex interactions.

If you wanted a creature to appear to be more intelligent in these adrenaline-pumping situations, make the creature hide its intelligence. Make it even more intelligent than it appears, but have it intentionally hide that intelligence. Then, in adrenaline-governed situations, it falls back on these intelligently created mental structures to make decisions. This would make it appear to be less intelligent in normal situations (when, in fact, it was far more intelligent), but in rapid shifting environments it reveals its intelligence. This makes it appear more intelligent in these adrenaline-junkie situations, when, in fact, it was actually less intelligent in them.

The effect would be like playing a Chess game calmly until move 24, in a position that looks dead drawn, when things suddenly get exciting and they make rapid moves, responding to you almost as fast as you can move the pieces. By move 30, you're in checkmate. It looks like they got smarter when they started moving faster. In reality, they had already checkmated you by move 24 (or perhaps even move 20), you just didn't know it.

Adrenaline's effect is temporary and would not help your hypothetical race.

Adrenaline does not allow you to take in more details during a stressful event. It only increases recall of such events after the fact.

Extensive evidence indicates that stress hormones released from the adrenal glands are critically involved in memory consolidation of emotionally arousing experiences.

Further, the effect wears off:

Do stress hormones also enhance memories of experiences that are not emotionally arousing? The findings of recent experiments suggest that this may not be the case. As discussed below, we recently reported that the endogenous glucocorticoid corticosterone enhanced memory consolidation of object recognition training when administered to rats that were emotionally aroused by an unfamiliar training apparatus. However, the treatment had no effect when administered to rats that had extensive prior habituation to the training context in order to reduce novelty-induced arousal.

Adrenal Stress Hormones and Enhanced Memory for Emotionally Arousing Experiences