2008 front bumper on a 2003 performance pkg?
 

Will it fit? For sure?

Yes, should fit without any issues

. . . except for the fender liners.

I just read another thread saying the 03-05 bumpers and the 06-08 bumpers are slightly different therefore they will not fit..?

As I said above, except for the fender liners, and some guys don't even change those, although that's not ideal. The side reflectors are also not the same, but that doesn't stop the nose bumper from fitting.

Yes, the 06+ bumper mounts and fits on 03-05 models however, as mentioned, there will be a smalls gaps on the bottom corners where the fender liners meet the bumper because the 06+ bumper is a little wider in those areas.