I run a K1 with aftermarket Y pipe, high flow cats and the bungs out and it is very loud. I'll get a reading at Castle Coombe for you all.
With the High flow cats i cant put the bungs in as it sounds really crap and raspy but nurrish has the bungs in on his with sandard cats and it sounds great.
As rtbiscuit says, I think it would be really really raspy with tes pipes/decat