The AS modification was for increased motor performance, especially at high altitude.
If I recall correctly the designation DB605 AS(M) is as follows:
DB 605 A - Motor, was used in the Bf 109G series.
S - Sonder (Special) Supercharger from DB603
M - MW 50 (Methanol/Water 50-50 mix)
There were other designations as well:
DB605 ASB(M) for B4 87 octane fuel + MW 50 (Methanol/Water 50-50 mix)
DB605 ASC(M) for C3 100 octane fuel + MW 50 (Methanol/Water 50-50 mix)
The DB605 D, used in the Bf 109G-10 and K-4 series, had two common variants, too:
DB605 DB - B4 fuel, MW 50 standard equipment
DB605 DC - C3 fuel, MW 50 standard equipment
As Ringleheim stated above the Supercharger is from the DB603, thus Sonder(special) to the 605.