I only have experience with my Borla but 98MustangGT just recently purchased the Dynomax kit from
www.classicdesignconcepts.com and is in the process of putting it on. I would think the Dynomax setup would be louder but the Borla has a nice mellow, aggressive tone and all the pipes and muffs are nice T304 stainless steel so they're gonna last. The Dynomax has a single oval tip on either side while the Borla has dual angled tips on either side. There really aren't any power gains with just a catback but I would imagine the two setups are comparable. Every little bit helps in the long run. Mine fits great and I have had zero clearance issues with it even though it's lower than stock. I think Roush might sell their setup as well. Follow my sig and look at the pics.
Brett
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66 Tahoe Turquoise Coupe
Laser Red 96GT Ragtop
30 Years & 30 inches apart in the driveway!