RE: What type of exhaust - Can/Pitts/Tuned Pipe?
The stock exhaust is still ok on the 100 cc but you cant use a horribly loud prop. I had stock DA exhaust w/ mezjlic 27x10 prop on my 35% H9 extra260 last year and scored 5 on the sound score some times 10s never had any zeros. most the time i was under half throttle except on 45° and 90° uplines and even then didnt have to go to full power. The plane was 27.5 pounds. Having the extra power help a lot. If the plane is IMAC only then tuned pipes are great but if you want to fly 3d with them you will have to get used to the lack of transition.
Im in the 40% planes for my imac now and run a DA170 on re3 tuned pipes and mis the perfect throttle transition of canisters or stock exhaust but on this sized engine it gets to loud to use stock exhaust. My current plane is to heavy for canisters so the tuned pipes extra power stays, but on my future plane that will be under 40 pounds i will use canns for the transition.