Realistically you can probably expect a 15 rwhp gain from long tube headers on an SRT. You will probably get better results on an R/T because of the restriction caused by the stock exhaust manifold.
I have american racing headers with high flow cats and mbrp cat back on my srt and could not be happier. It is an amazing combo, sounds great and i picked up a little power along the way. You would not be dissapointed in that combo.