I'm curious why you chose the 8.4's. Everywhere I had looked, sources recommended the 9's for Shimano levers. I paired the 9's with my old (but still functional) 5600 levers and have been completely pleased without using a Travel Agent. If/when I get mini-v's for my SS, they will be the 8.4's, as I have SRAM levers on that one.
Well, that explains it perfectly! Hope they perform well for you with the new shifty bits.
The trp brakes are meant to work best with the most recent generation of shimano levers which have a different pull ration then the older models. I have a set of 8.4 matted to new generation shimano levers and they are great but the same break didn't feel very good with campy or older shimano levers. Some people use the travel agent to make them feel better.
I have 9-speed Shimano (older). I stand by my assessment that they are vastly improved by a travel agent. When I upgrade my shifters, I'll post an update.
I spoke with TRP direct and they said either would work fine with my Ultegra 10 speed levers - 9's for more power and 8.4s for more modulation. I've been having terrible problems with judder and squeal with my TRP CR 950s. The 8.4s should arrive tomorrow so I hope they'll be OK.