Considering the Germans were going to put the DB 603 in the airframe for production.....the test aircraft for the line outperformed the 109K and G, as well as the 190A5. Tank took the DB603 and redesigned the 190 into the 152 instead, when they realized they could make 3 109's for every 1 G.55.
Also, the type was introduced in 1943. At that time, it was the best axis prop. Its performance wasn't matched until the 190D.
Again, debatable in 1943, not later. It was already matched by the C.205. You forget it was not a fast airframe. The 205 was faster than it already, and out before it as well.
The type was slow, it wasn't good at altitudes where allied fighters were escorting allied bombers.
While it was planned ahead to try the DB603, this would likely never have happened. They also "planned" the DB603 in a lot of things. That did not make the engine show up any sooner. Even the Me410 was planned to have the DB603, but never did, not even in 1944. The Italians showed a lack of infrastructure to produce their own engines quickly enough. Look how long it took to get the DB601 clone up and running!
It (the G.55) could manuver better than a 190. However it wasn't fast or high enough to take the fight where the 190 and 109 were going.
If this, if that, if then, maybe it could have been a contender. As-is it wasn't better than anything across-the-board. It had desirable characteristics, and that is why one guy on the German side wanted to build it. However that was not an official statement nor was it ever in the plans for Messerschmitt to work on them. It's all conjecture. Going off the actual performance of the actual production model (not a 1-off test plane) it was nice but not worth the time and effort for no real speed gain.
Where it WOULD have paid off was in the closing days of the war -- where they could have packed larger and larger engines in without changing the frame as much, where they could have packed more and more guns internally to kill bombers, etc. That again is all what-if, though.
As it stood, as a production model that saw combat, it was nothing great and nowhere uber. It is a sweet ride, though. I would like to see it.