I too have the Hartzell composite prop on my RV 7 (not flying yet) and have similar 1/4" clearance.
A while back I received a letter from VAN's telling me to check the clearance from the trailing edge of the prop blade to the cowl with the prop at coarse pitch before first start because someone with the same setup (in South Africa, I believe) reported that the prop struck the cowl when they cycled the prop. VAN's suggested that if I had the same interference I could send the prop to Hartzell to have the coarse pitch stop adjusted to provide more clearance.
I suspect that the composite blades will flex a little, but upon power application, if anything, they should flex away from the cowl. When braking they could flex toward the cowl, but then they should be in fine pitch with plenty of clearance.
I'm thinking 1/4" is enough, but I haven't started the engine yet, so I too would like to hear from the experts.
RV7 Standard Build - First flight 4/16/2016 - Now Flying!