For HE, according to my calculations:
4 HE factories (2x +50% 2x +75%) can quite nicely support 10 launchers using about 3.7 power/s, a bit less than a medium reactor (4.5 pwr/s).
So something like this would make a very width, cost and power efficient module:
I've not been able to get it to fire all launchers 100% reliably, though I expect with some finagling it can be done using moving walkways or missile storage etc. That's my challenge: make this or something similar (4 HE factories. 10 launchers, 21 width) fire consistent synced volleys (using testing method described in post above).
Here's some test ships in case anyone would like to fiddle: