No problem, glad to help.
I use 15% fuel. I either mix my own or add oil to store bought fuel. I have found that if you go to the website of the fuel manufacturer they will list the oil content. 3oz of castor per gallon should be plenty, since it is about 2 1/2%.
The red jett muffler should gain you about 1,000 to 1,200 rpm. It is basically a quarter wave pipe, so it is tuned for a particular rpm. You have to prop the engine to run in that rpm range. In flight you can hear the engine come up on the pipe. It is not the boost you get from a full length pipe, but it is pretty close and you do not have to tune it or find room for a full length pipe in the plane. I like them for a lot of applications, but I still run full length pipes when I am serious about speed. Call dub and talk to him, as he makes different color pipes tuned to different rpm ranges. The red muffler is the most popular and probably the best for your application.
For 15% you will not need a head shim. You can try more nitro, but near sea level you will need to lower the compression pretty soon. You may be able to get away with 20% with a cold plug, listen for detonation on the ground and check the plug every run. I would probably not push this engine that hard. They are pretty comfortable around 14K on 15% and will last a long time. Is it really worth a few hundred rpm to risk your engine? If it is, there is more in it. You can open up the crank, and raise the exhaust timing.
One more thing. Buy a good set of bearings from Bocca since the OS bearings do not seem to last very long. Keep an eye on the liner, since some OS engines like to peel the plating off the cylinder.
Scott