For the DB-601Aa, the critical altitude, no ram, was at 3700m (5 min Kurzleistung, thicker continous line), engine making 1100 PS there. At 6 km it makes 825 PS. Ie. the FTH, or critical altitude, was 800 m lower than for A-1, and 300 m lower than for A.
The Aa ('a' is for 'ausland', ie. export engines) was sporting a reduced capability supercharger (vs. A-1), so the altitude performance was not that good as in A-1. However, since the supercharger was also drawing less power to drive, more power under full throttle height was available for the propeller, eg. 1100 PS at 1,6km vs. 1050 (5 min rating), or 1175 at sea level vs. 1100 (1 min rating), Aa vs. A-1. The DB601Aa beats DB-601A, however.
We can see that, at FTH, the exhaust thrust at 600 km/m was to add another 10 % of power equivalent.
IIRC the Aa engine found the way to the Luftwaffe, too?