IMO the impairment of memory and cognition that most experience during tests is a result of your body's Survival Stress Reaction mechanism kicking in in response to a perceived "threat". When this happens your sympathetic nervous system mobilizes and brain activity is diverted from your fore-brain and its logic and reasoning centers into your mid-brain and it's more basic emotional, animal functioning centers. This process causes cortisol to be released into your body (as well as other hormones including adrenaline etc) in preparation for fight or flight. Unfortunately punching someone in the face and running out of the room will not help you pass a test.
My point is that reduced cognition etc. has more to do with your perceived level of stress and the parts of your brain that you are able to access during periods of high emotional arousal. The hormonal stuff happens as a result of this process but is not the cause (although it becomes a self-perpetuating loop to a great extent). Autogenic (4 count) breathing will provide a more powerful direct effect on your SNS and stress response than PS will. Breathe in through your nose for 4, hold for 4, out through mouth for 4, hold empty for 4, repeat. 5 sets will usually have a profound effect on your state of arousal, even in a genuine life or death situation.
PS won't hurt at all and may, in fact provide some immediate benefit, but your ability to manage your stress is infinitely more important. Supplement wise I would tend to recommend theanine more strongly than PS in the short term as it is shown to have a significant, immediate effect on anxiety by boosting alpha wave activity in the brain. Just my $.02. I hope it helps man, good luck.