Converting your large engined car to LPG will save you a serious amount of money - and even with average mileage, you should recover the cost of your investment in the first year. And LGP retails at around half the price of petrol and diesel, so every mile you drive after that, you're making a significant saving.
Here at V8 Autogas, we are authorised installers of AC Stag LPG systems, which are really high quality. AC Stag invest a great deal of money every year in research and development, resulting in their product quality improving year on year. That fact gives you the reassurance you need in order to invest in a conversion from us.
In terms of performance, you'll hardly notice any difference between LPG and petrol. While a litre of LPG holds a bit less energy than a litre of petrol, most drivers would need to look at the LPG switch to see which fuel the engine is running on to know the difference.
As an example, here's a video of a 2010 Mercedes E63 AMG - LPG Converted Everyday V8
[Thanks to Pendle Lease for making the video]