Anything is better than the stock restrictive exhaust manifolds.I installed the JBA shorty headers and the car definately breaths and sounds better.I anticipate you will gain 10-15 hp.
Once you hold an R/T exhaust manifold, examine it, then compare it to the SRT8 clamshell header or aftermarket header you will know what I'm saying.Worth the expense in my books.
Yes you will notice some improvment. The SRT Headers flow very well for a stock unit. Some guys here are pushing some big hp #s and still running stock headers.
I may have a set left with very low miles pm me if interested.
I believe you would have to use a 3" to 2 1/2" adapter from the SRT clamshell headers to the R/T mid-pipes.I'm sure I have read this on the forum previously.