05 5.3L crew cab....losing power?

[michigan2500hd]

I have 05 1500 crew cab 4x4 with the 5.3L, 3.73 gears. I have been driving about 125 miles one way for work lately. After doing this trip so often i have started driving a little faster then i used to, can you say boring,lol. On the expressway i used to go about 70-75 mph. The last coupke of trips i got behind some cars going between 80 and 90 mph and the truck seemed to really be working hard to keep up. Even other trucks were passing me doing every bit of 95 or so. I have a westers tuned pcm (87-91 octane tune), magnaflow muffler, k&n drop-in ( still clean and has not been oiled for several months). This truck should cruise along at these speeds much better then it does. I will change back to the stock pcm and airfilter before the next trip and clean my maf sensor. I wonder if the magnaflow has something to do with it? Maybe it wont let the exhast system scavange the gases correctly. Anyone has any idea?

p.s. i know slowing down would help,lol.