its tapers pruely for the reason of back pressure. Spoon does this so they maintain mid power. Otherwise if your looking at drag racing ur car, u want the top end so the bigger un-taperd exhaust will help in this case the downfall being losing some lower end power.

Kinda like if u want the 4-2-1 headers for low to mid or the 4-1 for all out top end.

I belive honda have taped down some of their exhausts around the header flange to aid a bit of back pressure but i could be wrong. I believe vtec engines love to breathe so if you are custom making it use mandrel bends and as few bends in it as possible. If u want it quiet put one or two resenators in it but ull lose a bit of power otherwise dont run any and it be massive loud!