H-Pipe Questions

I'm planning out my H-pipe and I've got some left over 2" but the exhaust is 2.5". Do you have to use the same diameter for the cross tube to get the most out of it or would using something a little smaller be okay?

2 inch H on a 2.5" system is A-OK! Many times shops do a poor job of sizing the hole for the crosspipe (they get lazy and leave it way to small)--just make sure your openings are as close to the H pipe diameter as reasonably possible.

And sometimes they do garbage like this.