I think I can speak to that. When I swapped the 2.2 liter B20 into my PV I initially used to original exhaust system. This was the single downpipe manifold connected to an exhaust system using tubing that had (I measured it at the tailpipe) a 1 1/4 inch inner diameter. Even at idle the exhaust would be blowing out the tailpipe under a fair amount of pressure. The engine's power would just sort of slowly taper off past 3500 rpm and be mostly gone by 5000 rpm. This all immediately disappeared when I put on a header (IPD style 4:1) and 2.25 inch exhaust. As far as I could tell (via butt dynamometer) it didn't affect low end torque at all, but now it had the usual 'D' cam torque curve, and would pull strongly up to 6000 rpm.

BTW - that header is a bit of a PITA. Very prone to leaking at the manifold gasket. I'm going to try two gaskets, and if that doesn't work I'll just replace it with an FI manifold and downpipe, perhaps a 4:2:1 Svedish header.