Kirkham side exhaust

[Dimis]

Power wise you won't know or feel any difference.
The flow rates I'd guess would be almost identical.

Theoretically the stainless steel may last longer as it deals with corrosion & heat cycles better, but in my experience the steel last plenty long enough, particularly if you paint or ceramic coat them.

If you want best performance improvement for $3k I recommend spending it on 18-inch wheels & some decent rubber