The M50 manifold losses power below about 4000 RPM, increases it above 4000.
When we put a Turner Stage 4 kit on our M Roadster (S52) we kept the stock manifold since the car is street driven only.
For a track car, I would put the M50 manifold on.
'95 Alpine M3 LTW
'00 Dakar M Roadster
'02 Topaz M3
Red/White SRF #4 (Chassis 561)