If you are doing ws at that low of tp, you aren't using prime weapons properly. Tp scaling can not be surpassed by more buffs. You aren't capping dmg lower then 2.5k on any weapon I've used or seen. Also, if you are talking in terns of shear dps out put, you wouldn't be using mp. I am not saying one shouldnt but the fact of the matter is, casting takes time and time limits tp gain / ws frequency/ dmg output.
Depends on buffs there is very small DPS difference between using Origin at 1000+ TP doing 80-90k damage and doing one more attack round to cap damage. Also with enough buffs like SV Aria, absorb STR/INT (with Liberator) and high enough Scarlet, you can reach 99k or very close with avg TP of around 1400+Moonshade /sam and much
I also doesn't suggest to use magic mid fight with bosses. Nether Void + DS Absorb STR with Liberator and bis set has over 7 min duration. You can do it on trash mobs while doing objectives and have it up for boss 5 min later. When there is no trash mobs stops before boss, you can still do that absorb as a pull spell on the boss at start. I haven't tried it on all bosses, but I think it should land with Dark Seal, just with half resisted duration (still long enough for entire fight). Endark II can also be nice and you cast it before fight.
I can not accurately compare the dmg of the GS to the Scythe as I do not own either but I do run with a friend who has stage 4 scythe and I out dps him most times using caldbolg (when I am on drk of cource). The tp gain is slow and try to make up scenarios where it is superior all you want but at the end of the day, it's just not there.
With optimal buffs, but without Savagery sheet suggests Caladbolg and Foenaria stage 4 to have very similar dps alongside Helheim stage 5. Only Stage 5 Foenaria seems to be ahead in this scenario, so it wouldn't surprise me for Caladbolg to beat Foenaria stage 4 in practice, especially with random factors like luck and player skill.
With optimal buffs and Savagery, Caladbolg is significantly ahead of even stage 5 Foenaria, until you change Foenaria setup to 3hit build, then Caladbolg is only slightly ahead.
