The only people that seem to get excited about the 3-stage manifold on the N51 are the people with the N52 that want the 3-stage manifold. My experience has been that most people think the 3-stage manifold only exists in the HO versions of the N52 and so they're surprised that the N51 has it also.
The N51 does have a slightly different power curve than the N52 with what feels like more mid-range and top-end power. Of course, it could just be that since you can feel the manifold changeover points (3,000 and 5,000 RPM, I think), it's placebo. If you do some research on the N51, you'll find that it's rumored that the N51 actually makes slightly more power than the N52. But since I don't have any proof, that's just a rumor.