There was a small modification starting with the A-6, but the extent was simply reorganizing the outboard gun bay to accept the larger magazines and guns and internal stuff for the later outboard armaments (MG151/20s and Mk108s). It wasn't larger and it wasn't lighter and it wasn't an improvement (not aerodynamically, I mean).
Also, the 190A8 never had GM-1, and GM-1 will damage an engine if run below 30,000 feet or so. It's an extreme altitude boost only.
You may be thinking of MW50....

However, not many A8s had this installed. Many used the MW50 tank to store regular fuel. This is where the AUX tank in aces high comes into play. That's why the Dora doesn't have an AUX (it's holding MW50 instead).
The 190A9 may have had MW50 in small numbers, but overall it wasn't big for the Antons.