I had last year a LPG SAN 210 and this year a LPG SAN 230. I also run 3 RangeRovers and a Volvo Turbo on LPG. I don't really notice the powerloss on the boats. But I do on 1 Range Rover, but that one is a little bit different setup. My Volvo Turbo is even a little faster on LPG.
We use the light weigth tanks on our boat. There fit 6 of them in the rear compartements. You can wakeboard 1 hour on each tank.
I think a normal conversion is around e2500-e3500,- . And then you can still drive on Petrol.