did your mental functioning improve while on 200-300mcg selenium/day?

I didn't notice direct effect on the mental performance. Indirectly, though, it did help clear up the flue-like malaise which itself saps the energy for any work, mental or physical. Since in my case that phase of heavy immune response was transient, the six months during the removal of amalgams and 2-3 months after, I don't need it all that much any more.

There were couple supplements that did show dramatic effects on mental performance and perhaps the folks here might wish to give them a try.

The first one is Huperzine A (maintance dose 2x50 mcg/day 6 hours apart; I use Swanson's brand) which raises the levels of neurotransmitter acetylcholine (by reducing its breakdown rate). Subjectively, when I took it the first time in the "attack dose" (4x50 mcg at once to test the range of effects), it felt as if I was up to that point looking through the fogged up eyeglasses which Hup A wiped off perfectly clean (max efect is 1.5 hours after taking, the positive effects last 5-6 hours). The thoughts become much sharper, with a kind of cold clarity to them, and it is effortless to follow up the longer chains of thought without losing track. The good aspect of Hup A is that in the longer run it stimulates neural growth and branching of neurons, which ought to enrich the positive effects over years. I experimented with nearly all brands out there and settled with Swanson's due to producing the most consistent effects.

The second supplement is Acetyl-L-Carnitine or ALCAR (2x500 mg/day). It provides dramatic mental and physical energy boosts, you just want to get things done, you don't wish to put anything off and no effort seems too hard any more. I also noticed a perceptible boost in libido (others, mostly males, have reported that, too). Similarly to Hup A, in addition to short term benefits, it has the long term rejuvenating effects on brain and muscles. Unlike Hup A, with ALCAR I didn't notice any difference among several brands I tried.

After about a year, I didn't notice any down-side with either (other than missing the positive effects if I don't take them; there is no craving, though). From the literature at the links above, it doesn't seem they have any negative side-effects.