I've never seen any definitive answer to this question but from a purely subjective viewpoint difficulty does make some difference to scoring but it is not that strong a function.

Purely as a guess I would say that the difference between suicidal and cakewalk scores are probably on the order of 3 to 1. This is assuming that someone played the same game at cakewalk that was played at suicidal. By that I mean won in the same timeframe with similar values of income, population, etc.

For more realistic ranges of difficulty, say between crippling to suicidal, my guess is that the differences in scoring are minor. Certainly there have been top 25 and top ten players that play at obscene and masochistic.

To answer your question I see no reason someone playing tough or above couldn't crack the top 25.

I had seen a post on this as well. My memory tells me it was adjusted by multiplying, not to the power of. It was an adjustment of .1 per level, but I don't remember which level was 1.

I.E. If Suicidal was 1.5, Obscene would be 1.4 and your score would be (Game Score*1.5) for Suicidal. Not a huge bonus given the differences, but something.