From my personal experiences, I have noticed the same problems with putting Flowmaster mufflers on any of my cars/trucks (except my 98 Z71). Their mufflers are built to support a bigger small block and big block apps. With all the new BS that's integrated on these new V8's, the back pressure is dropped at some point in the system.

I don't know a lot about the new GM V8's but I know that what I see more than anything on the 5.4L Triton V8 (Ford) is Magnaflow and Borla. Flowmaster is on its way out as far as I'm concerned.

Don't know if it helps at all, but I don't want to make you feel bad