I run 3" to the rear axle with Dynomax Ultraflows.

Pretty quiet inside the car. I do have carpet but no insulation.

I haven't tried with/without testing at the track, but I can't imagine it being very restrictive.

'63 Dodge 330
11.19 @ 121 mph
Pump gas, n/a, through the mufflers on street tires with 3.54's. 3,600 lbs.
10.01 @ 133mph with a 250 shot of nitrous an a splash of race gas. 1.36 60 ft. 3,700 lbs.