Every car is different when it comes to taking in mod's.. I would imagine these guys who do "mail in flash" are experts in tuning this exact engine.

The mail in flash due to mods such as intake, exhaust, etc.. is a generic map created from another G35 with the same such mods.. which it can't benefit you to the very bit of limit.. is pretty close enough and fair to boost hp and tq on your car also knowing it will be safe for the engine.

Honestly.. the people who do mail in flash I would look into there reputation on mail in flash's from other people who got them.. and they wouldn't cost as much custom dyno tune.

After you get a mail in flash take it to a local dyno and check it out..