I doubt that claim....that number is way too high. Maybe 10 Horses or so but 28??? Dyno itI am getting mine done next week also.
I'm going with the 22" magnaflow & going single 3" all the way out. ( just so you know, he tells me the stock size is 2-3/4".)
He claims i will not lose any low end from this & should see about a 28 hp & torque gain. All for only $300.