They tested MW50 and found the speed gain to be 16km/h at 100m. Given the complexities of the MW50 system, they opted to go for c3-system and higher manifold pressure rating at the time(not just A8s)
the C3-injection system basically raised the fuel flow with an added pump. It was said to be especially effective at improving rate of climb, which made it a viable option for the A8s as well. The degree of usage is unknown though. However, it WAS used.
By Juli 1944, all A8s were rated with the heightened emergency manifold pressure(no C3, no MW50, no GM-1 explicitly stated in this context), which bumped the speed by 25km/h at altitude(1,65 ata).
It was activated by a pull lever on the main dash.
Source: http://www.deutscheluftwaffe.com/archiv/Dokumente/web/new%20site/frames2/Dokumente.htm
I have also included the drawing of the proposed MW50 system for the BMW 801-D Engine, as found in the A8.