I used to have a 2000 D2 FL S8 on LPG. Was a Prins system and was excelent. The main real difference was in the wallet. Cleaner and hotter burning which is nice. I found power to be only a little down and only when you really booted it, and it would switch back to petrol after a second anyway....Found it did it a little more when the tank was low, so i'm guessing it was a lack of presure/flow.
And one time the LPG filters needed cleaning, which was an easy job.
To be honest with a 4.2L engine, loosing a little top end power is neither here nor there, you can switch on to Petrol if you find a nice B road to blast, but most the time 95% of a 4.2L engine is more than enough
Always starts on Petrol and i'd advise keeping a 1/8+ in the tank as if the petrol runs dry it can fry the petrol pump and petrol is the lubricant. You also tend to drive more in a if there is an LPG station with reasonable prices you top up, rather than letting things get low. Had a 90L tank across the back of the boot, sure it took up space, but the boot is cavenous that you had a normal size boot left anyway.
My d3 A8 4.2 has a wheel well tank and it's about 60L which is a bit small in my mind and things rattle arround in the boot more now!