another possibility would be to decrease the amount you are flashing. if you use less power, it will cycle much faster. if you decrease to 1/64 power, and just use fill, you should be fine to shoot bursts.

also this may be a viable idea because remember, a lot of the time you are relying on the venue's lights and set-up to give a lot of the colors and light of the photo, because that's what makes concert photography more natural. If you're flashing the hell out of the performers, it just won't look natural. Oh and 1/64 manual power on both of those speedlites still gives you a good amount of light.