LPG power loss??

Impossible to answer, depends on engine, kit used etc etc. Some applications can result in pretty much no power loss, some 20%. What car are you thinking of converting?

Also for high powered cars, under full load the LPG may switch over to petrol.

I would say power loss shouldn't be an issue, in that you can always switch to petrol if you want to drive in a more spirited way, and most of the time you can leave it on lpg.

Generally if fuel economy is an issue, you'll end up driving in a more spirited way with LPG, enjoying the fact that it's costing you so much less than on petrol.

My last car, i just switched over to petrol when i fancied a bit more power.

Bit like the M button on a M5....just with a few hundred horses less!

Couldn't really tell any difference between Petrol / LPG in everyday use, just when i fancied a thrash

LPG powered cars can actually give more power than an equivalent petrol.LPG has a much higher octane rating than petrol, hence can take a much steeper advance curve in the ignition. The octane rating reduces though as you warm it. Other advantages are LPG does not wash the oil off the bores like petrol, and does not dirty the engine oil either.

It depends. LPG is about less 10% energy rich than petrol, so you lose about 10%. That loss is mitigated by better burning characteristics, which can recover 3 or 4 percent depending how smart the ECU is.

On a decent setup the difference isn't really noticeable.

it depends on the car and the conversion, the new conversions are pretty good the old ones less so.

I have had the following LPG/petrol cars;

Volvo T5 - quick car even on LPG but you could feel some loss not much, newish system.
Mini Cooper 1.6 LPG - new system, made almost no difference gas or petrol
Golf 4motion V6 - old system and was still a quick car but you could feel some power loss on switch over
Galaxy 2.0; was a modern system but felt slow (even slower) on gas than petrol
Focus 2.0; old system felt a bit lethargic on gas
Volvo V70 factory fit LPG 2.4 n/a; a little slower but nothing to complain about
Astra 1.6 dual fuel; factory fit this was not very good, unreliable and big drop in power
Zafira 1.8 aftermarket new system was as good on gas as petrol , very impressive.

Beware of the older systems as I have found when they go wrong the LPG specialists are not interested in fixing if anything more than a simple problem - their solution = replace with modern system.