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.