So have a few hours on my new 12, planned on a pipe from the beginning just out of habit but after riding it I think about all I would like might be some more over rev.  She sure doesn't need more power and throttle response seems very crisp, engine tone is good and visually the head pipe seems over diameter already.  At first I was thinking of the fmf full system with megabomb header 4.1 RCT but don't really like how much it sticks out, probably burns a lot of pants.  A guy had one on ebay for half off with 4 hours on it, we chatted and he said it gave more low end grunt but took some off the top end and he didn't like it so was looking for another system.  Any real world experience out there and what specifically it did?  My ktm had full akrapovic system and it woke it up everywhere.  My 09 crf450 had MRD full sytem and it made a big difference on that motor with overall power, crispness and over rev.  This bike I don't know for sure if it needs anything.  Considering a slip on in the fmf 4.1 RCT in anodized blue with carbon end cap (in part because it looks sharp - not a good reason I know) but I mainly want a bit more over rev.  I was told it was the megabomb head pipe that added the low end but I don't know a lot about pipes.  This is the best stock pipe set up I have rode for sure.

I just got a FMF powercore 4 because I needed a spark arrestor...I don't notice any power difference...only thing I noticed was the sound, less raspy and more of a deep bark...

I wouldn't mind a little more over rev myself...

I'm in the same boat and looking for a pipe that will smooth out the mid and bottom coz it really is quite brutal at times. Aparently the pro circuit does this with a slight sound reduction too!
Let me know how the FMF goes as that is slightly cheaper than the pro circuit...