Have you considered going the diesel route? Quieter and if you route the exhaust to rear of the plane with plastic or aluminum tubing, it is quieter still and your plane will be much cleaner. Your .15 won't be much louder than an electric.

You won't lose any power - in fact you will gain with the ability to swing much larger props (even a 10in. prop is not unreasonable on .15 size diesel).