That extra gear would be nice for highway cruising, but i could definetly not justify the cost. If anything happens with my driveline it's gonna be fluid changes for differential and tranny. My LSD has some wicked skipping sometimes so I'm hoping some decent fluid will clean that up.
P.S. As long as your pulling the transmission, you might as well just go to M3 for it, bigger transmission, bigger driveshaft, bigger differential... you get the idea. However I'm not sure if it will fit on your engine