Sorry to hear it didn't all go according to plan sawdust, particularly as you bought the SB-600 specially for this shoot
You did get some good shots tho', so very well done there
Might take a long time + precision work, but could you p'raps fix the noisy ones' BG's only without running the whole image thru' (thus affecting subjects too)??
Might not seem like it now, but you will look back + laugh about this one day ... when you're on your 100th kickboxing photo shoot
Good luck for the next one