BorlaXR-1 are supposed to be good, power and tone wise (I've never owned any, and if I've heard them, I don't know when), but are expensive.
Almost every muffler test I've ever seen , dynomax ultra flows win for best power and moderate noise volume.
Of those two, I'd use the ultraflows. The biggest longest case that will fit under the car.
(For what it's worth, I'm currently using 3.5 in/out hooker aeros, 3.5 flowmasters, and 3" magnaflows on my cars).
Engine masters did a 1000hp muffler test. (4"). Hooker max flows won (over borla and two others), cheapest, most power, middle of the road noise.
