My first thought was; if the manufacturer is saying it needs to be balanced, I would listen.
The more RPM you turn, the greater the balancing factor has to be. I don't know enough about "balancing factors" to explain to you, but if it was me I would be on the phone with the guy who builds it.
My personal experience is with an open Mac 91. Horstman used to sell tungsten counterweights that you welded onto the crank. If they helped, I couldn't say for sure, but I qualified first in every race I ran that motor, in Norcal, and won all but one race. Clutch failure.