I use to own a taxi buisness (untill i got divorced LOL) and most of the taxi's these days are straight gas and the ones that arent dedicated gas get run on at least 1 or maybe 2 tanks of petrol every week...............
also they dont run a whole tank of gas without turning the car off in a taxi you turn the car off after every customer so they get stopped and started hundreds of time a day way more times than your average private car that only gets driven a couple times a day.......

To the OP.

Considering you can't really do much with the intake side of things due to the gas mixer, you need to look towards your exhaust to increase power. Depending on budget, if it's not much, start by removing the rear muffler & possibly the small 1st muffler if you have a completely stock system.

I found my EB was still fairly quiet with only the middle large stock muffler, stock manifold, cat & pipe size. Made a good increase to my performance & economy for bugger all cost.

Currently the EB has a set of pacemaker extractors, 2.5" commodore cat, 2.5" straight thru muffler, and a 2.5" resonator up the back. It's still fairly loud, but legal, and has a nice little idle note.

To the guy saying about seeing more heads failing from gas injection, it sounds more like a lean tune would have caused it, and yes, many lpg setup's are tuned lean for economy reasons! I have a LPG Injection setup on my EB, it's almost 5 years & 180,000 kms old, running pretty good. Only thing i've had fail a few times other than the head gaskit, was the distributor.

Cheers

ToranaGuy