OS55 and pipes:
I have tried the following
Align 50---I had it on hand when I got my OS55 and per Bert K's recommendation for the "standard" 50 pipes at the time last fall, was the best choice. I agree with that, it tunes well, makes good power overall, can be a bit "peaky". Runs great in cool weather. When the motor is tuned well, the motor sounds a bit raspy and almost like it's lean, when it's not.
Funtech A395(OS powerboost)- Runs similar to the Align, other than a nicer "throaty" sound. Has a bit better torque and maybe a wee bit better top end power. Nothing night and day though. The sound though seals the deal for this over the Align 50. May need a spacer on some airframes to clear the tank, prevent it from hitting the tank/melting it :O. Not a deal breaker, but keep in mind when ordering so you are not p/o'd that you have to wait for another part.
Outrage 55--Good overall muffler. Cheap and crash warranty is good. New backed threads on the flange are a nice addition to prevent stripped threads. Sound is good. Somewhere between the A395 and Align 50, but not annoying like the Align. I don't mind the Align sound, but some complain about it, so thought I'd mention it. Power wise. Maybe a bit more top end and a bit less torque vs the A395. Overall flying goes to this pipe though, but neither is compelling enough to switch out if you already have the other.
Hatori 566--Seemed to make good power, probably similar to the Outrage 55 overall really, except pricing. Quality of course is Hatori, so no worries about how it will hold up long term. Price is high though and for me, and my "crashing" style lol, I would probably fold it up before I could get my $ out of it, so I didn't stay with it.
Funtech B320(ordered should be here next week)---Spoke to a couple people I know and trust in the hobby. This is supposed to be the cat's A$$ for the OS55. Can richen the motor several clicks and make better power while running cooler.
I'm eager to try the one I have on order.
My main knock against the OS55 now, is how much hotter you need to run it vs the YS56. The YS tunes opimally for power and comes down at about 180 or so just under the head. My OS's make good power when tuned optimally, but run MUCH hotter. Easily into the 210-230F range or MORE depending on humidity and ambient temps. At least with my cheapie temp gauge which could be off too. HOping the Funtech B320 can help hold the temps down a bit while adding power.
I don't want to get into a huge debate over temps b/c I don't tune by temps. I do like to know what they are occasionally just out of curiousity and I think there is some correlation between temps and lifespan of internal parts. My OS's are holding up great, no appreciable wear despite the higher temps over the YS. Just an observation on my part and a potential concern for my paranoid mind to ruminate on hehe!
Randy!!! I am the liquor