Well, we don't have any idea yet (as far as I know) about how difficult the final boss mobs are going to be. According to Matsui's chart, the current T4/5 Delve NMs are at a difficulty level of 14, while the final bosses are at 20. I imagine accuracy on them will be quite a different matter than current comparisons.
Without accounting for accuracy, the multi-attack on Thuamas is worth roughly +17% to overall damage (both melee and TP for weaponskills).
A fully upgraded Manibozho body will have (vs Thaumas):
10 str
15 dex (possible crit rate)
22 acc (including from dex; possibly more from the next patch)
37 att (including from str; possibly more from the next patch)
If the accuracy matters, 22 acc will be an 11% hit rate. 11% hit rate is +17% overall damage when base hit rate is 65%. So if your native hit rate is 65% or lower (actually, 65% relative to Thaumas, so 59% without any body accuracy at all), Manibozho automatically wins, since it also adds in fStr, attack, and probably crit rate (would guess 2% just to be conservative, but potentially a lot more).
Since you can expect the Aphotic Kukri to be pretty much required in order to fight the final bosses, fStr from 10 str will be gain something in the 2%-3% range.
37 attack vs a mob with 600 defense (assuming reasonly high defense, but also reasonable amounts of def down applied) is worth about 6%, though applied to only half your damage, so another ~3%.
Given the gains from fStr, dDex and attack are likely to be fairly universal, we can then adjust the point where the accuracy puts this above Thaumas. This works out to 87% relative to Thaumas accuracy, so 81% without any body accuracy. For simplicity, if your hit rate without a body on is 80% or lower, Manibozho will beat Thaumas.
Since that's all theoretical, going to try to put that comparison into the spreadsheet.
Delve boss: 600 defense, 530 evasion
Hit rate without body: 79.5%
DPS with no body: 348.780
DPS with Thaumas body: 434.615
DPS with unaugmented Manibozho: 453.825
~~ dDex was too high; pushed through the sweet spot; increasing boss agi to negate that
DPS with no body: 343.341
DPS with Thaumas body: 427.725
DPS with unaugmented Manibozho: 428.905
DPS with augmented Manibozho: 450.784
Clearly I underestimated the difference. Even unaugmented, Manibozho ties with Thaumas when base hit rate is at 80%.
Since augmented Manibozho is hitting the accuracy cap, I just need to reduce mob evasion until Thaumas hits 450 DPS to find out where the cutoff is.
Thaumas hits 450.514 DPS when hit rate with Thaumas is 91.5%. Thus the cutoff for hit rate without including the body is 85%.
This is always subject to a variety of other considerations, but I would still consider that as a reasonably valid inflection point between the two options: 80% for unaugmented, 85% for augmented.