I asked a similar question a while back and got very little response.
I just replaced a fairly new FMF power core with a FMF Turbine
core 2 unit and besides being noticably quieter and let tinny sounding,
the bike has much better torque .
If it wasn't for the original KDX silencer being " stock " , I'd say to replace
the pipe and use the original Kawasaki silencer.
I'm not sure what the db's of a straight thru silencer are but unless your racing
or have a very highly modified motor, you don't want a Power Core 2 .