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Can I suggest that if internals are stock (or close to stock ie. standard cam/tb/heads/etc) he would probably be better off to look on ebay as you can get exchange ecu's programmed with mafless tunes, from repeutable companies. The ecu is sent to you and you plug it in and you send your old one back. Its a very cheap alternative, as there are some on there for like $300, unless you have a cam/heads, etc where the tuner will customise the tune to suit the mods, thats when you will want to fork out the money to get it done properly and accurately.
ebay tunes are good for your basic mods such as exhaust and otr.
Check it out, as the listings tell you all you need to know inc. dyno sheets.
No question is stupid if your willing to learn, a mafless tune is where they tune your car to run without the maf (mass air flow meter), as the maf restricts the amount of air going into the engine. By removing the maf you usually put a otr (over the radiator) cold air induction which allows the engine to breathe alot better, giving you better performance, and in some cases better fuel economy. The mafless tune uses other sensors in the manifold to calculate and do the same job that your maf did.
It can be done to the 3.8 ecotec and it would probably work well with LPG as it will probably give you more power than someone running standard unleaded.
Just no, a dyno tune is always better. Don't no how they can show you dyno sheets of cars with remote tunes, every engine is different and will make different power levels.
To the op, $600 sounds good for the tune. Russos is a good shop too.
LPG won't make more power than unleaded unless it's a Liquid Injection system. Highly doubt this guy has a performance orientated system on an everyday road car.