I just finished my Bane costume this morning, thought I’d throw some info out in case it helps:
I started with the original Bane being 3-50 with max 8/8 skill. I used 1* same-color feeders (which is my habit for 3*) - each gave 450XP on the costumed version rather than the usual 180XP I would see from a 1* same-color.
I fed one-at-a-time, and Bane Costume reached 3-50 with only 4/8 skill. I then topped it off using 40x 1* same-color feeders (ten-at-a-time for 100% skill increase).
Final products, now that both original and costume are maxed and 8/8 skill, including the corresponding +5% Attack, +5% Defense, +10% Health:
Costume version has +37 Attack, -7 Defense, and -80 Health compared to Original. 280% Special damage vs 300% but the difference in Attack rating makes those almost identical, with Costume having -40% Blind instead of -35%.
Conclusion: I will most often use the costume version if I’m making attacks (I only use 3* in Rare events and in 3* tourneys), because the special will hit for about the same damage, but the tile damage will be higher; with a dip in durability being the tradeoff. But I think the tile damage will be worth it.
EDIT: +5% mana from the costume bonus seems useless on this hero, as in cases where I would use him, I would not have a mana troop available to bring him to 15% mana which is what he’d need to get to 7 tiles.
Good gaming!