I would say that exhaust systems (other than stainless vs mild steel) are cosmetic and performance ones vs standard provide little if no performance improvements but my make elevated noises.
The Barchetta engine and Hitachi ECU combination have shown themselves to be pretty damn good and I don't think anyone with drastic cam shaft changes, mods etc. has really extracted and real performance improvements.
So for exhaust systems you can rule out so called performance increase data and focus on longevity (stainless vs mild steel), tail pipe look, and sound.
We replaced our "B"s system quit a few years ago with a "longlife" custom built stainless system. Not too expensive and is certainly well appreciated etc.
Just want to add/stress again that the Barchetta / Punto HGT 1.8 / Coupe 1.8 engines are renown to be very difficult to performance upgrade and STOCK has been the best overall performance one can get in a road going configuration.
For reference back in 2011 we had this Longlife system fitted:
New Longlife Stainless Steel OE Exhaust - made and fitted by Matt at FastFit Service Centre Ltd., Basingstoke
Tail Pipe : TP911LH
Middel Box: 1703002
Back Box : 854102&1/4