hmm i would of thought all ECU's could be remaped,well like i said a custom re-map might be better but you would have to get another ECU.
a intake would not give alot of power,i am not up on SRT but even the best ones dont seem to pull for than 20bhp at the very best of times.
do you have a cold air feed?,5bhp sounds about right for a cat-back.
when i mod a car i usaly remap the ECU,fit a inducution kit with a cold air feed and then put in a full exhaust system
and then go from there,they seem to be the cheapest way of getting the most power.
and NOS is pointless unless you are at santa pod IMO,i would use the money else wear,get a turbo KIT (full kit)
i bet you could get to around 550bhp depending on the turbo and PSi
and just enjoy the car fella,it will be quite fast ATM and its a nice car anyway