[Aussie Mike]

If you build the engine to suit LPG you will probably get no real loss in power. In fact in some cases you may even gain power. LPG has less BTUs than petrol but it has a much higher octane rating. This means the fuel itself doesn't have as much energy but the higher octane rating lets you run a lot more compression without detonation.

The newer LPG injection systems have some extra advantages. Liquid LPG injected into the port turns to gas and gives a big temperature drop to the incoming charge making it denser. It's often observed on gas injected engines that they get an increase in torque over the same petrol setup.

That being said it's difficult to compare setups between windsor and LS1 in a discussion like this. There are so many variables in how each engine could be built. What I will say though is that either motor running on petrol or LPG will make more power than you know what to do with in a light weight Cobra.

Cheers