250-500cc Wr300 euro pipe

I found a new wr300 euro pipe for cheap and was wondering if it would be ok to use on my built motor. Guy said there is a cat in there. Would that cat affect performance?

If it has a cat just cut it out. I had one and just cut the flange off, there were a couple of spot welds holding the cat in place, knock them off and just weld the flange back on. There was also an emissions pipe which I cut off and welded the hole up. So far as I can see the pipe looks otherwise identical to the non-cat pipe the bike came with.

Can you leave the cat in there? Will it really affect performance?

I guess you could, but it is fairly restrictive when you look at it. It's easy to get out so I don't see why you would want to leave it. If it's for green reasons you would probably have to leave the emissions stuff on there as well. I know when all this is left on the bike won't rev at all and produces the same power as an electric tooth brush!