OK, this is my experience of an LPG conversion, and may not be the experience everyone has!
Given the choice over, I would never have had it done. Powershift grant? Forget it, the only Lexus eligible is the GS300 for some reason.
My car was the first IS200 to be converted by the company involved (parent firm AND franchisee). I opted for a full size tank due to the mileage I cover (minimum 100 miles per day).
The car has never run right on gas ever. It ticks over too low, occasionally stalling, after a long time at high speed (e.g. a motorway run) as you slow down it often cuts out or has no power (particularly annoying when your trying to pull onto a busy roundabout), the gas consumption is horrendous (12mpg) making it cheaper to run on petrol. Even when running "correctly" the car is a little sluggish.
My advice is: don't let them experiment on your car, wait till they do it to someone else's then test drive it.
If you'd like, I'll e-mail you the details of the company I used so that you can avoid them like the plague.