Performance differences are negligible,
we are talking hundreths of a second,
when comparing acceleration statistics,
between petroleum and LPG.
Economically though, LPG gives on average,
40% better economy compared with petroleum.
me dads 164 cloverleaf runs on LPG except for the times where somebody tries to go faster than he does.
the difference is quite noticeable especially when its top end performance you need on the motorway!
you get a bit less but £20 for a full tank nobody can complain!
My V6 156 also runs onb LPG - excewpt when someone is trying to go faster than me! The acceleration difference is certainly noticable and i would say that my 2.5 on LPG is like a 2.0 petrol.
LPG (propane) has a higher octane rating but a lower calorific value. That means no knocking/pinking but it also means that weight for weight it creates less energy than petrol during combustion.
I have just completed 75K miles on my LPG and would say that its saved me about 1/3 the cost of that milage (including the cost of the conversion). Thats quite a lot I guess!
However, if its fun and power you want choose petrol! As a suggestion to make a 1.8 go faster, depending on your money situation, get a supercharger fitted and make sure the suspension and brakes are tweaked too. Mmmmm