The cheap ones are exactly that, cheap. Some fit, some don’t. Hence I don’t bother selling them for a G40 anymore.
“maybe but surely it's got to helpful before that point? as opposed to when it becomes the limiting factor?”
What most people seem to forget is to tune for the overall goal. If that’s 160 or 170hp, putting on a massive manifold on with 2.5” downpipe and exhaust may let it scream more top end, but will be detrimental to torque lower down on the rev range when that is the car’s best attribute.
I’ve seen many Germans with G60 laders and stock exhaust manifolds pulling big horsepower. In fact I met one the other day claiming 265ps on a G60 lader’d G40 on a stock exhaust manifold. Regardless of whether he’s had his g40 RR’d at the club’s favoured Aldon, if it’s 20 or 40hp out, that still shows there is nothing wrong with the original manifold for anyone running a G40 lader. I’d be intrigued to see if the ppp manifold would make any power on that setup. That is the type of power levels you’d want a 2.5” downpipe in my opinion, and that of many books.
A quick google shows people claiming to have made 400hp on a 2.5” exhaust - so why are people fitting them to 160hp polo’s?