His honesty isn't what i was questioning, i've just known a few garages to say something that sounds good/technical to gain some time to find the fault, in fact in many garages it's common practice, especially on a job where the final fix doesn't justify umpteen hours of labour to find the fault fixed with a 50p part.
I always thought Jetex were heavy duty aluminised mild steel? I've been looking at an
OBX Racing stainless steel exhaust. I'm sure they do one for the B234 engine as well. Although it has to come from the USA, with the exchange rate etc it still works out cheaper than Jetex and if i've linked to the correct seller they will ship to the UK (quoted me about $160 to ship a pair of V6 manifolds and de-cat pipe, reckon a Volvo system would be about the same) so might be worth considering as an option?
I wonder if your car would still have passed the emissions test with a higher fuel pressure? Fuel injected (mixture) is a function of fuel pressure and injector time so the higher the fuel pressure, the richer the mixture for any given injection time. Likewise the injector time could be extended by the ECU to compensate for low pressure but that would only extend so far until fuel starvation problems show up.
As you have probably worked out, as manifold pressure changes the fuel pressure should alter as well. If your pump can only provide 2bar max pressure on the unregulated side then there could well be a problem there.