The heat output from a single shell is 7, assuming PGI stays with the canonical heat output.
Ghost Heat from the standard AC/20 is +11.52
Double-tapping both UACs simultaneously would be 7 + 7 + ~12(GH) + 7 + 7 + ~12(GH) = ~52 units of heat.
Firing both UACs without double-tapping (which is what I was talking about) would be half of that, or ~26 units of heat.
Clan ERMLs generate 5 units of heat apiece (per TT), so four of them (assuming they share the standard ML's GH limit of 6) would together generate 20 units of heat.
With 18 DHS (assuming the same HS scheme remains in place), it could vent 3.12 heat/second.
With 18 DHS, the heat scale - that is, the heat threshold - would be 30 + (10*2) + (8*1.4) = 61.2 units of heat
(
source on how HS & heat scale work)
Heat-wise, such a
Daishi as I have described would be quite capable of handling the heat from a single salvo of double-tapped twin UAC/20s
or a single salvo of single-tapped twin UAC/20s plus a quartet of ERMLs (again, with the second being what I was talking about) on a neutral or even slightly warm map.
Very few 'Mechs outside of the fastest of the Lights and the
Cicada (which, contrary to popular belief, is in fact a Medium

) are likely to evade the first two shells (which will land in the same general spot), and there is a good chance on most 'Mechs that the quartet of laser beams would mostly expend themselves against the same general area.
Alternatively, double-tapping the UACs means a second front-loaded 40 units of damage going toward the target, with a fair chance of hitting the same general area as the first set.
Moreover, that spare 3 tons would be enough to get it up to a 340 XL if the Engine isn't locked - enough to boost its speed from 48.6 kph to 55.1 kph (or 60.6 kph if the 10% bonus from Speed Tweak is included), in addition to boosting its torso twist speed.
Granted, it's not as ridiculously powerful as it would be without Ghost Heat and with all of the DHS being "true-dubs", but it is far beyond what the vast majority of 'Mechs currently in-game can do (though, the x6 PPC/ER-PPC/LL/ERLL/LPL
Stalkers come close).