Actually, engine thrust is added, two engines equals twice the thrust output of one engine. This doesn't achieve twice the flight performance compared to just having one engine, but that's irrelevant here as it's just about thrust. The reason the GAU-8 causes negative acceleration when used in real life is because atmospheric drag is helping it, that's why I tested it while being stationary, eliminating this variable and making it all come down to thrust.
This means that, while there is decceleration when firing in real life, the decceleration while firing in game, even when you're actually flying, is more than it should be.

Regarding your Mighty GAU-8 mod comment:
Engine forward thrust isn't really added. A bit, but not like 2 engines equals twice thrust output. The GAU-8 INDEED creates negative acceleration on the whole A10. It's still that kind of an beast. Therefore the mod is pretty accurate ;)

:)

Wie auch immer er das geschafft hat...

http://steamcommunity.com/sharedfiles/filedetails/?id=792892691