They will be very loud - trust me. Any 3" straight-through muffler will be a lot louder than a 2.5" muffler.

The flow specs listed look like they came right from Dynomax - or they were tested the same way using 20.3" H2O as the gauge. To the OP, if you have 550 HP & were running dual 2.5" Super Turbos, they will provide approx. 380 loss-free horsepower, so yes they were restrictive to an extent. The stainless 2.5" x 20" bodied Ultra Flows will flow 1075 sCFM, & are rated at 978 loss-free HP in a dual system. I also believe they will not be too loud - they may not be much louder than the Super Turbos until you are all the way in the throttle. These would be my first choice for a stock-looking muffler setup if I wanted an oval muffler & maximum flow. Most of the guys in the Pure Stock drags use Ultra Flows. If you have room for resonators in the system (maybe back in the tailpipes?), you could add those also for extra noise control if you need it. I can build you some in any length if you need something compact & effective.

Another muffler I really like the looks of (internally), are the Cherry Bomb Vortex mufflers, except they are built like boxes. Internally however, they have a really close design to some I designed a few years ago & I've heard a few sound clips with these mufflers installed & I like the way they sound. Plus, they will flow very well although I've never seen flow specs on these under the same type of baseline testing we did at Dynomax.