This came up in a thread recently, and I think it will be useful as a part of the FAQ. A comparison of different 5th gears vs. final drive ratios, showing their respective RPMs at higher speeds:
(Stock 16V)3.67 R&P, 0.89 5th =
3715rpm @ 70mph
3980rpm @ 75mph
4245rpm @ 80mph
Top speed(5th gear x R&P x 6000rpm) = 117mph
(4K)3.94 R&P, 0.85 5th =
3809rpm @ 70mph (+94rpm)
4081rpm @ 75mph (+101rpm)
4353rpm @ 80mph (+108rpm)
Top speed(5th gear x R&P x 6000rpm) = 114mph (-3mph)
As you can see, the difference in RPM is negligable, and will still allow for passing without downshifting to 4th.
(4K)3.94 R&P, 0.80 5th =
3585rpm @ 70mph (-130rpm)
3841rpm @ 75mph (-139rpm)
4097rpm @ 80mph (-148rpm)
Top speed(5th gear x R&P x 6000rpm) = 122mph (+5mph)