Interesting reply from FMF. It seems to me that they are in the bizness of selling spark arrestor/silencers. So, I wouldn't lend much credibility to their reply about another company's product.
Well, I have some experience with Cobra's Sparky Spark Arrestor. I used on for years on my KTM 250. The Sparky is a screen type spark arrestor, and you should experience no power loss whatsoever when the unit is new and clean. The screen is contained in a module. As you ride your bike, the screen will accumulate carbon on it. As it becomes clogged, you could possible notice a decrease in power.
The screen module needs to be cleaned on a periodic basis. I knew guys who cleaned the carbon off by heating the unit with a torch. Personally, I'm not a torch guy. I used to just soak the thing in a gallon of carb cleaner for a few days (during the week) and it would be clean and good as new.
Needless to say, this spark arrestor should do exactly what you want it to do. The only think that it won't help you with is sound output.
FWI, Cobra was a sponsor of team Suzuki Off-Road in the early 90's. Hawkins used to run the sparky welded onto an aftermarket silencer on his RMX. I don't however remember if he was using FMF or PC at the time.
Hope this information helps you with your decision.