Even small glucose levels can apparently have significant effects. I have some papers in my library arguing that the memory-enhancing effects of adrenaline (which doesn't cross the blood brain barrier) are mediated by the glucose increase it causes. One of them demonstrated that a glucose-mimetic molecule also acted as an enhancer. Overall, the data seems pretty convincing that getting a suitable dose of glucose is enhancing, but the effect has an inverted-U curve - there is an individual and task dependent optimal level.
Overall, drug responses are very individual and we shouldn't expect enhancers to be "one size fits all". For me, modafinil + sleep deprivation produces a state that feels like sleep deprivation but is apparently quite functional (as measured by my ability to write software). Mileages may vary, indeed.
Transhumanists have high hopes for enhancing human cognitive abilities in the future. But what realistic steps can we take to enhance them now? On the one hand Flynn effect suggests IQ (which is a major factor in human cognition) can be increased a lot with current technology, on the other hand review of existing drugs seems rather pessimistic - they seem to have minor positive effect on low performers, and very little effect on high performers, what means they're mostly of therapeutic not enhancing use.
So, fellow rationalists, how can we enhance our cognition now? Solid research especially welcome, but consistent anecdotal evidence is also welcome.