V10 F-250 exhaust mod. Loss of hp & torque?

Hello all, I just joined the forum and am exited to see the things I learn on here!
I have an 03 f250 6.8l v10. The truck is amazing, great for towing my 23ft gooseneck horse trailer. I have a tuner, airbags in rear, reusable high flow filter, and just upgraded my exhaust myself. Why do this stuff? I want to get the most out of my 6.8l as I can without doing anything big to the engine to gain more power for my heavy loads.
As stated above I just put a flowmaster, two chamber muffler on it and A larger diameter y pipe at the bottleneck. I know the thoughts on back pressure for torque, some say it just moves the peak up the rpm range…idk. But, with my new freer flowing exhaust, did I lose anything? I figure the stock manifolds and stock cat will give sufficient back pressure to not lose anything, but am I wrong? I love how it sounds but getting up to 60 from a dead stop on a grade is my demon in the truck. To make things worse I have the 3.73 gearing. I want to make sure I’m not losing anything with my exhaust…please advise? In hindsight I wish I went with the 7.3, but this is what I have and I do love it.