FPS limits have never changed. 410 FPS guns have really never been turned away from 400fps games for as long as there have been chrono. 420fps has simply formalized what was already in place (and maybe raised it 10fps).
What I've witnessed though is that while AEG limits have become firm (I think no one is being let run full auto on 425) people can still get away with a lot in the DMR category. Just a few weeks ago a guy said he was running 470fps as a DMR. I also know people have been let through chrono with 470fps guns that are full auto capable and just told not to use it.
My personal opinions are this:
a) 420fps won't go any higher for full auto.
b) I don't like the DMR category at all. I don't think that it is safe because you can put out a lot of rounds really fast in semi. Most semi aren't exactly waiting a second between shots.
c) certain AEGs FPS vary a lot depending on temperature and humidity. Bring your own chrono to a game and compare if you want. I think it has to do with temperature impacting how well the air seals are performing. I don't think chronos at games are inaccurate. Sometimes your gun will perform up to the internals you put in it and sometimes it will not.
PS: I ran 399fps this last weekend and do not care what the limit is as long as it is a HARD limit. I do not want to match the standard exactly but go up against hotter guns in the field that I could have matched had I known I would have gotten away with it.