You need to use the correct plug.. And as far as i know the only one that you can use is the NGK IFR8H11.
But as far as why its choking and popping in the top RPM range it could be the main jet is a little rich, you could try going down a size or 2 on the main jet and see if it helps..
Well then its not the fastest in its class... When it came out it was, because there was nothing else in its class but now the LTZ400 is around and it is a lot faster. Plus the new one is EFI with MUCH better suspension then the 400ex.
If its not backfiring and its just popping its not really a big deal, and pretty normal. Most 450r's do that, if you really don't like it you can put a richer pilot jet in though. (that's what i did, stopped the popping that i hated so much..)