It depends. The 370z sport package comes with an LSD, but I don't have the sport package, I have the Touring package.
30WHP actually is a notable difference, and I'd net even a few more horses when I get my car tuned that I wouldn't get if I tuned it stock. The LSD would definitely be the best choice from a racing standpoint, it increases stability by a ton and cuts time off my 0-60. I was considering getting a 370z manual pumpkin and putting in 4.08 gears, but that'd cost me a grand with the stock LSD and I don't like buying things used. It would, however, cut more than a half second off my 0-60 times.