could i get some numbers for 4-5x monolith 12''s vs 2x Monolith 13s''?
Here you go.
This assumes a 4dB SPL summation factor per doubling of subs so five subs adds 9.3db over a single in the above calcs. In practice this summation factor will vary by frequency but should be in the 4dB-6dB range if they are time-aligned but not co-located (i.e. stacked).
So the dual Mono 13" option should have more extension and probably a more explosive delivery quality from having greater dynamic headroom from fewer subs. The key advantage of using more 12" models is if you need them to eliminate nulls in a particularly difficult room.
I'd take four of your current Mono 15" models over any of these options. You already have enough SPL capability so even just adding a third 15" sub in the right location may be enough to knock a tricky null on the head.