I've done a little exhaust system testing, your 2.5 system is not what I would stay with based on my personal experiences.
Car one is my 1969 Dart GTS with a stock stroke and bore 440 motor with a set of the old11.5 to1 TRW pistons and a set of mildly ported big valve 906 heads, TM7 intake and early production Holley 950 HP carb, Hooker 1 7/8 inch fender well A body headers with 2.5 inch pipes through a set of the Thrush 2.5 inch Corvair turbo muffs with a set of 2.5 inch tail pipes exiting in front of the rear wheels. That system was around 7 ft. long and weighed maybe 25 lbs. per side with the muffs, head pipes and tail pipes
The car ran from 12.31 to 12.23 at 112.+ MPH in runs in three hours at the old LACR track on a test and tune Friday night with the exhaust system on.
I remove the pipes and muffs and made the next run the next morning with about 300 Ft higher difference in the corrected air density, first run with no other changes it ran 11.39 ET at 117.+ MPH
That shock the heck out of me but it repeated near those times(11.39 to 11.41 ET) the rest of the day with the open exhaust
I got beat in the 4th round and put on a set of 3.5 inch Bullet type muffs hooked directly the header collectors and it slowed down to 11.54 at 116.+ MPH
I thought it would pick up by adding collector length, but it didn't
My last pump gas car got a 3.0 exhaust system on it with those same Hooker 1 7/8 inch fenderwell headers and a set of the old Magna flow 14x7 inch oval muffs fed with 3.0 inch pipes from the 3.5 inch header collectors with 3.0 side inlet and center outlet size muffs mounted at the rear bumper. That car had a 517 C.I. Pump gas stroker 400 motor in it with a low deck six pack set up with stock type 440 vacuum carbs on it.
I raced it here locally at our 1/8 mile Madras track with the exhaust system hooked up, it ran a best of 7.01 ET at 95.3 MPH. I remove the full exhaust system after racing that afternoon and the next run was made at 10.30 AM the next day with less than 200 Ft air density differences, it ran 6.98 at 95.5 MPH with no other changes
Not worth taking the exhaust off, huh
I would make the entire exhaust system bigger I.D. if I was you