As a GK main from day one in vermintide:
Virtue of the Joust is completely fine. No need to make a second FK out of him, honestly. It helped me countess of times when reviving/holding my ground/clutching, and it has amazing synergy with one shot greatsword même build. (Not only greatsword! Any weapon that loves push attacks benefits from it a lot, such as mace+sword). Considering latest balance with better push range on Shields, i think its reasonable to leave this talent be (30% stamina recovery).
I like the stacks idea giving 30% power and splitted in 5, good idea from ons balance. Strength pots literally save me sometimes as well. Boss ledging, kiting, clutching, all benefits from having them.
Completely agrée with the guy above about merc, hes completely fine and has various builds depending on weapons and talents.