I've been developing a larger site, which will probably amount ot ~350 to 425 pages.

Is there any magical number of pages not too exceed, before diminishing the rankings or other effects that could happen?

I think its more important how you structure the site and use internal linkage to make sure the most important pages are given the most weight.

there is no magic number really. although you need more links to get more pages indexed if you are going to have an exceptionally large site.

An optimizer once told me about some basic rules for Google in place giving the recommended minimum # pages per term/keyphrase on the site, what % of times keyphrase/keywords should appear on the page per total (not to exceed) and # of variations per keyphrase that needs to be present throughout the site. Ever heard of such as study?

Ever heard of such as study?
I think some of the algorithms probably change based upon query type. Additionally they evolve and certain markets should probably be approached in different ways. Odds are such a study was used to position its writer as an expert to sell their services, but I do not know that it would have much practical value.