I've had my mk4 converted now for two years and very pleased with the set upI have which in Prins.
Try not to go for the fit in one day company's.It is a difficult job as there 's not to much space left to put the kit in but can be done usually the front end will need to be dismantled.
You will also need the valve saver kit.
With the multipiont injection systems now fitted rather than the single point as of the old days,power loss is now a myth.
Just thought I would add about MPG...That will dipp slightly depending on how heavy you are on the loud pedal.
I just worked out my real world(not MFD display) MPG from last fill was 22.5mpg.
Fuel cost - 60p litre
Spend - £36.40
Distance - 300miles
Welcome to the forum DAVEH. It's coming up to 3 months since I had my Mk5 converted. Its a late March 09 registered R with relatively low miles.
I run the R as a daily and am getting 24 to the gallon of LPG. I get no reduction on power as Dodders has mentioned. The injectors are installed as close to the intake as possible having seen the front off during its installation.
I agree with Dodders too in that it is worth going to an installer that takes their time, and I had the flash lube installed for the valvecare on my Prins system too.
I have a 50 Litre tank that sits in the spare tyre well and I reckon I will break even in 14 months. Gas is 65p - 67p up my way.