You probably needed new spark plugs? And so when they put on the new spark plugs for LPg you got "more power"? Could be many things, maybe your sensors weren't acting as well as they should've been, or your car was in desperate need of a tune?
The system may have been adequately tunes, good computer used etc. LPG has potential if it is known how to be worked....... Also, the injectors may have been shagged, therefore LPG running better than petrol. Many variables possibles. You will find out eventually.