Look - I think the problem you have got here is that LPG reduces power by 15 - 25% - first issue.
Second, there is no spare wheel well so where is the tank going to go?
Third - you lose the lubrication properties associated with the petrol system - really not good in a car like this. You also junk pretty much all of the warranty in one stroke. Also, I daren't think what it would do to the residual value.
I get fed up with the number of trips to the filling station and the associated cost but, unfortunately it comes with the territory. A more economical car is the answer if it really doesn't suit.
Financially when you take into account the conversion cost, the warranty issue and the residual issue it cannot make sense.
It doesn't make sense from an RS3's drivers point of view either.
Looks like we were typing at the same time ED... Just took ages for mine to post due to terrific net speeds in the middle of the ocean!!