It is possible to restore the power on post A4 models by replacing
the exhausts and the carb tops.
Models A1-A4 produced around 110bhp.
This was reduced to a mere 92bhp with the introduction of
the A5. Don't believe people who say you won't notice the
difference and that the A5 is plenty powerful - it is NOT
and you WILL.
The exhaust on early GTRs had
a bigger exit hole and produced more power. I believe a Neta
exhaust restores most, if not all, of the power lost by the
smaller exhaust (but check before buying).