It looks like Vishnu gained 13 rwhp and 28 lbs rwtq from welding straight pipes in place of the cats on a stock midpipe, with no tune.
http://www.m3post.com/forums/showthread.php?t=585325
I am sure there are some dynos around of "name brand" catless midpipes, with no tune. I would think that nice mandrel bending and maybe a better X section would be worth a few rhwp, though I am not sure how perfect the X needs to be in order to have the desired balancing and/or scavenging effect.