I believe it would be worth it. Just a few points of speed can determine who doubles who in a fight. I honestly would sack a +2 for a better IV Sothe. Better to do this at +2 though, if it were a +7 or more, I would just stick with the Sothe I had.
You keep everything (except SP), including prior merges, so you can always change IVs. You can also check the resulting unit before confirming the merge.
If you merge that +2 into a new 5 star sothe the new one will be +3. If you really want some assurance just grab a few 4 star units you have a bunch of and do a test.
Pokemon GO
Fire Emblem Heroes
Fate Grand Order
Dragonball Legends
Dragalia Lost
IV Calc
Heroes