Not a doctor... observed some interesting side effects with my parents on these, and would personally err on the side of caution with them. If you do come off though, do it gradually - also there are different types, so it may be worth looking into that.

"would he have been sharper without them"

Probably not? It'd be pretty unusual for an already sharp person to become sharper at age 80.

Sharper at 80 with statins or without statins was the question, not sharper at 80 than at 70.

Since he kept working for another 19 years, including supervising heart surgery on Boris Yeltsin, he probably didn't suffer much of a decrease in sharpness on statins, if any.