Peter,

Sounds to me like you need the ability to generate more air/ground speed. i.e. higher gearbox ratio and a smaller diameter prop. I encountered the same problem with the GWS P51 when trying to fly at 7000+ elevation. I ended up gearing at 4.4:1 with a 9-7 prop instead of the recommended 5.33:1 with the 10-8 prop. Keep in mind, a larger prop with a low gear ratio generates a high static thrust but it does not mean it will move through the air very fast. I smaller prop turning faster may generate the air speed you need to ROG. A higher airspeed is how real aircraft overcome a high wing loading.

Good luck!
Prop Wrench

Also, If the model weighs only 8oz, you may consider a Speed 300 with about a 3.7:1 ratio. They will almost perform with a 400 but are significantly lighter.