You likely won't get any mileage or power gains - the best MPG (and power gains) will be had from keeping a clean air filter, ensuring your 02 sensors are functioning well, using good (93 Octane) fuel, keeping your tires inflated, and practicing mechanical mercy by accelerating and decelerating calmly and avoiding rapid throttle inputs.
That said - any quality exhaust manufacturer (Magnaflow, Flowmaster, Borla) should produce a good sound - it's the nature of the Rover V8. You may have to remove the resonator upstream of the muffler to ensure the best sound - but any exhaust shop worth their salt should be able to fab up a decent section of straight pipe. Keep in mind that this mod would cosmetic only, and having to listen to constant exhaust drone gets old REAL FAST. Not to mention that the cops love to give fix-it tickets for overly noisy mufflers.