A well tuned LPG built the same as petrol system like bolf has said multi point inection and mapped fully will out do a petrol car any day of the weak as said earlier its equivilent to running a higher ron fuel.
If your rebuilding ans SR to run 400bhp ie injectors ECU and re-map, its going to be only slightly more expensive to do it on LPG needing a fuel tank aswell. and i'd bet the lpg one would do better. It's something I plan to do anyway but not on an Sr
