I had a Whirlwind RV200 on my RV8 with two E-mags and got a kick back during start, which broke a couple teeth off my starter ring gear. Fortunately, the starter wasn’t damaged. I ended up resetting my start timing to fire at 5* after TDC. If you do that with your SureFly, you’ll also carry that 5* offset throughout the advance curve. Max advance will be 5* less than design, so maybe 30-32*, which might not be a bad thing. Your after start fixed timing will also be offset, so your 25* engine will be a 20* engine. I don’t think you would be getting full takeoff power doing it that way. I talked to the folks at SureFly about this, and they told me to just set it as described in the install manual (at TDC). He said they haven’t experienced problems with kickbacks if installed per the manual.
My engine builder (JBA) told me the kickback could also be caused by my improper installation of the sniffle valve on my forward facing sump engine, which didn’t allow all of the residual fuel to drain out of the intake manifold area. Somehow this residual fuel can be drawn into a cylinder and fire at the wrong time. Our electronic ignition systems use a wasted spark, so when #1 is firing, so is #2. If that #2 piston is just starting up the cylinder and there’s the right combination of fuel and air, that nice hot spark created by your EI could fire causing a kickback. I ended up modifying my sniffle valve install correctly and also reset the timing, so I don’t know which one worked, but one of them did. Never had a problem again.