As far as I can remember (it's a long time since I flew a J31) only the earlier marques had WaterMeth.
In general I agree with you - if performance is a critical factor, then the J31 is NOT the aircraft to be operating. It doesn't like rough strips generally, and you'll find it problematical.
If performance is the critical factor, go for a DHC-6. If you need pressurisation and the strips ain't too bad, have you considered a Kingair?