RE: engine/fuel question
15% requires a more open valve setting; more nitromethane needs less oxygen to burn, therefore would be running lean with no adjustment from 10% settings.
The increase in nitro content is mostly about the ability to burn more alcohol in the combustion chamber, since you can reduce the amount of air and increase the volume of fluid (it has a lower fuel:air ratio).
The only thing I noticed when going from 10%-15% on a .46 engine was a decrease in flight time. I don't believe it is worth the extra cost and reduced flight times for the small increase in power you get.
The increase in power gets more noticeable when you get into larger engines.