There's also the issue that, unlike hullmods, the amount of free OP given by this would be mount size dependent. An eagle could get a maximum of 15 free OP (Heavy Needler), while an Apogee or Champion or even Sunder could get 30 (Plasma).
people are already only building in the most OP-costing hullmods which, let me check here, goes up to 15 on a frigate, which u start the game with & dont even need to look for & source for each one you mount. Whereas the highest OP weapon a frigate can fit on almost any slot its gonna get (small universal) is the antimatter blaster at 9, or even a medium universal for the, like, 2 frigates that get any mediums is the heavy needler at 15 (which AFAIA no frigate actually has the flux stats to use without completely specializing itself into close support & being very vulnerable to anything that can keep up with it which is
all other frigates). There's no way to argue that this change would actually seriously change the OP considerations for frigates. And going up in size from a frigate makes the OP gains from building in a weapon
worse, since the OP cost of hullmods goes up but weapons dont. On a destroyer, the most OP expensive energy & missile weapon tie out with hardened shields, and one of those is
the proximity charge point defense launcher and the other is the single most flux-expensive medium gun in the game, so unless you already wanted to use the heavy blaster you're better off building in hardened shields if nothing else, for OP considerations. Granted the heavy needler actually beats out everything but SO, so one out of the thus-far 8 use-cases for this is either a side- or down-grade. It does not get better from here. Your example of a ship that would be broken by this change, the most expensive gun it can sling is tied with or beaten out by
24 different hullmods. Even the single highest OP non-redacted gun in the game is
15 OP less than SO on a cruiser. On a capital ship its 20.
There is just no argument that this change would even affect the game's balance, except for the mudskipper.
& at that point we've entered the realm of madness & honestly the mudskipper probably should be allowed to have its gun for free
(PS, it bares mentioning that I was assuming in my OP that it was a given that you wouldnt be allowed to build in infinite guns & theyd count for the 2-3 buildin limit)
I don't really like it. One factor is that since you need to find the weapons first
you also cant build in hullmods until you find them & theyre rarer