I don't think there is much of a hp difference with high-flow cats it is more of a cost and maintinance issure. I personally like the sound of borla better than magnaflow but that is total preferance. you could always try the magnaflow and if it did not sound the way you like sell them and put some borla xr-1's on. exhaust is total preferance. to many people act like there are good and bad sounding exhaust when it is really just what you like. Like me I love flows on something I drive just for fun but If I have to cruise the freeway they anoy the **** out me. Some people just love them period. Magnaflows are almost too "deep" for me. I would replace your midpipe first with the catback you have now and then decide if you want to change. unless you have lots of mods 2 1/4 inch exhaust with free flowing muffler will do you fine.
I have a magnaflow catted x with borla turbos welded in just like you. I like the way it sounds; quiet at idle and screams at wot. However, I would def not mind a little louder. To be honest, when I put on the catted x ,the car seemed even quieter at idle but woke up a bit at wot. I was thinking about going with the packs but it would be a bit much for a daily driver. Well for my taste anyway.
I had the magnaflow pacs on my car with factory h and the magnaflow x-pipe. I sounded ok but too much drone and the tone never changed from 1,000 rpm to 6000 rpms. I replaced the pac's with Borla XR-1's and I love them. Borla rocks!!!! No drone on the freeway and sorry to say to all you pac fans but the XR-1's ARE LOUDER W.O.T.
I have never heard any stangs with Borla's so I might be biased. But, I have the Packs coupled to a Pro-Chamber and everybody tells me it is the nicest sounding car around. I think the only people that don't like it are the cops and the elderly. I love the way it sounds and it is even better when you see the looks on people's faces! The magnaflow crew will jump in here to convice you to get em.
IMHO, i remember about 6 months ago Magnaflows were new and no one had them. Everyone made fun of the Flowmaster guys for being all the same. NOw, everyone and their mom has Magnaflows. My friend has a Bassan Catted X w/ Packs and it sounds too weird for a stang. I have 40 series flows with O/R Bassan X and EVERYONE loves it. Im oging with new mufflers too btu ill be getting Dynomax mufllers to be different. Ever considered Dynomax, Spintech? Its ALL about personal preference. The difference is HP is very minute. No one goes with exhuast for power, choose by sound.
Personally, I like my magnaflow exhaust. You can get the catback for $300 and the catted X-pipe for $379 from www.fulllineexhaust.com