While I have no experience with the Webra 160, I do own a 120 and it flies great !!! ... It's an excellent engine.

The reason why it tells you to add a shim is because the comperssion in your Webra is very high already. You *might* not feel it, but being a ringed engine, when the ring expands it creates a lot more compression. You add a shim to lower the compression a bit because higher nitro fuels are more explosive (nitro supplements O2).

There's nothing wrong running 15% nitro ... you will save $$ and the engine will have A LOT of power. The reason why its designed to run w/15% nitro is because in Europe (where Webra engines are made) Nitro is very expensive so people only run 15% at the most.