Yan,
The muffler choice is critical to the performance.
I've flown many YS56 helicopters and have yet to find one that has more power than the OS55HZ coupled with the PowerBoostII or Hatori.
The Hatori is an excellent choice for a pipe, make sure you get the one designed for the YS60 as it breathes much better.
The Power Boost II is also an excellent choice and also sounds a little better than the Hatori. Make sure you do not get the Power Boost I as it was designed for the OS50 Hyper and will not work properly on the OS55HZ.
Thanks,
Matt