youre making entirely, entirely too much out of it.
yes resonators are tuned, but its not like youll lose 30whp if you just weld one in. you would be far from the first person to do it. theyre usually tuned for the physical pipe theyre on and the muffler selection. at most, it probably nets you 1whp and gets you closer to the manufacturers desired exhaust note. aftermarket catbacks have some tuning, but they dont scrutinize it to the point theyre considering "what midpipe is this guy on"... they get the most benefit they can from having an engineer or tech look at it for a few hours.
what kind of budget are you on, by the way?
and again, if i was making the choice for you, i would get almost any resonated catback, a 2.5" midpipe, and the CS downpipe - two piece or one piece.
youll have a resonator designed to work with the system, its pretty quiet, it will be slightly louder and deeper because of the improved down/midpipe but thats your best option, period.
i would not hack up a catback to add a resonator when theres better, quieter, and cheaper options out there. you know youll be happy with a resonated catback, but if you add one, its a risk. why take the chance?
options - OEM, racing beat, i think megan racing had one(?), and if you can find it, the greddy unit is outstanding. ive NEVER seen a greddy catback get sold on here in almost 2 years though, so good luck.