I'd like to know how these little engines work out. I have a 15cc JBA and after I replaced the carb I got it to run reliable, but it is rather heavy. So my .75 size plane does not perform as well as with the same size four stroke glow engine. It has lots of power, but the plane feels heavy.

So a 9cc would be installed on a .40 size plane (?). I don't know how much this engine weights, but I am sure it'll be a lot more then a glow engine.
Most planes this size have a hard time handling the weight. Then there is very little room for ignition box and regulator or ignition battery. It's always a good idea to install your ignition components away from the receiver, and servos.
I think there is a future for these engines, Enya just developed a gas engine which uses a glow plug, no box or extra battery. I think that'll be great fro the smaller planes.

For the planes 20cc and up, my planes all have gassers and I am happy with them.