Acceleration quirks, duration quirks, structure quirks. Pulse to duration, High Engine for the Accel, High Engine for the structure.
Using standard mediums would give you more alpha and another ton. Also a 390 frees up even more tonnage for armor at the cost of only 1 kph.
Going for pinpoint damage. If I'm using mediums, its for range. I want to crit the first mech I see within the shortest amount of time possible.