I've never done a before and after so I cannot tell you if there is any change in anything it's always been a fresh build on a new project, but the theory is there and I believe it so I run them.The theory is just like your 4 into 1 header the exhaust pulse from the previous cylinder helps evacuate the next cylinder.The h pipe creates a low pressure pulse from side to side, so the left side helps evacuate the right and vice versa. If you've got the pipe why not run one?
My last build bigger motor, better heads,lots of cam, single plane manifold,etc.etc gear change.Did the H pipe make or break that I doubt it, but it didn't hurt it.