My first choice would be to send them back and get an unbroken set. Hollowing them out would still be better than stock cats. I'm not sure what the size of the cat chamber on those pipes are, so you may be right. My guess would be that you'd never know- the difference would be small enough to be within the range of dyno variance.

I'd say 4". They might change too going small, to big, back to small. Who knows though.

I don't know either. It may be one of those things where it's technically "worse" and the difference isn't noticable at all in the real world. I'd still just send them back or see if the warranty was transferable.

I PM'ed you and the seller about the situation. He should be contacting you; just fill me in if/when the deal is done.