In the distant past (patches 3655 and 3750) T2/T3 engineers were given rougly same BP per mass as T1 engineers. making them as efficient as T1 in terms of buildpower.
However these values weren't adjusted to the mass storage stats of T2/T3 engineers. Currently all engineers give around 1 BP per 10 mass. 10.4 for T1, 10 for T2 and 9.6. Slightly better with each rier. With mass storage it's much less consistent and much worse the higher the tier: 5.2 per 1 mass storage for T1 engie, 6.5 for T2 and 7.8 for T3.
Suggestion:
T2 Engineer:
Mass storage: 20 -> 25
T3 engineer:
Mass storage: 40 -> 60
These values will make every tier of engineer to provide 1 mass storage per 5.2 mass invested in engie.