I have one question what was the estimated speed
Evidence for a DB 021-powered Me 262, while it does exist, is extremely scant. I don't know of a full project report on the Me 262 with 2 x DB 021. It's not quite the same thing but Focke-Wulf, in Baubeschreibung Nr. 281 Einmotoriges Jagdflugzeug mit PTL-Geraet 021, projected a speed of 905km/h at 8km altitude for a fighter equipped with 1 x DB 021.
Presumably Messerschmitt's figures for the Me 262 with 2 x DB 021 would be somewhere in the same ballpark. 905km/h = 562mph. With 2 x Jumo 004 B-1, the speed was supposed to be 900km/h (560mph).