I saw a dyno comparison with stock mufflers and with stock mufflers removed on a 2005 GT mustang. it gained 2HP and lost 3TQ, just telling you what i saw and know. They chalked it up to less BP. So not wrong.

 

you can make a dyno read what ever you want, ive never seen a track time go down

 

i just ran no exhaust for about a 30 min ride dam it is loud but the drone is really bad
i will tell you one thing, it gurgles like beutch when u let off of the throttle
i would not recommend running no exhaust with tips