Krusty, you get 1 more 20mm cannon, and a higher service ceiling in the G.55 vs the C.205 (41K vs 37K), but other than that the difference looks to be pretty small. The "extra ammo" I expect is from the 3rd 20mm cannon since you get 200 rnds in each of the wing mounted cannons on the C.205
The C.205 is a bit faster (399 vs 391 mph), but that might be the result of the extra weight. Both rides are using the same engine - a Fiat RA.1050 RC58, which is a clone of the DB605A-1. The G.55 has the greater wingspan, so I expect you are right about the high altitude performance, but the climb performance I am reading about leaves a bit to be desired (6000m in 7 min, 12 sec / under 3K ft/min which I know I can beat in the 205).
I'll try it out if is shows up, but there are probably much better rides out there to spend the modeling time on.