It could very well be a leak in the fuel pump assembly. On the 08+ you have to modify the housing and use new o-rings in order to get everything to seal up properly. If you don't do it right, you could end up with a car that seems to work OK, but will eventually end up having slow startup issues and even can go lean at high RPM's and WOT.
I had the same issue as you when I originally installed my fuel pump, and it went away after using two o-rings that were slightly thicker. A few months later the issue came back and I also noticed it was going lean at higher RPM's, so I pulled it back out and the pump was loose again, and I ended up having to grind off a little more plastic to get it to seat properly.
I would start by checking the pump and housing assembly. If you have a wideband I would also monitor AFR's. If you don't have a wideband, I would suggest avoiding going WOT until you get the issue resolved.
Here is a link to the thread I originally made: