I own a 2009 Malibu 247 with a surf wake that I love, so take this review with that in mind.
Rode behind the G23 twice with no NSS and stock ballast. The wake was plenty tall, and had lots of push, but it was generally much steeper than most wakes and did not have a very long pocket. We played with the ballast a little, but the wake didn't change that much. I am sure with more time, we could have gotten it dialed in more. Overall, it was a very good wake for nothing beyond stock ballast. Considerably better than my Malibu with only stock ballast.
Have a friend who has since purchased a G23 with NSS. With just stock ballast and NSS, the wake is, in my opinion, substantially better and more fun than the non NSS G23. The wake is the same height, but is less steep, has a better transition, more push, and is considerably longer. It is also very adjustable on the fly. This wake with only stock ballast is better than my friends old SAN 230, leaned, with an extra 750 in the back, and lots of people sitting on the surf side.
We are just starting to add weight to the G23 with NSS. Put two 250's in the back last weekend, and the wake definitely improved (got taller and meatier). Maybe 75-85% of what I get with my Malibu slammed and leaned with an extra 2300 lbs (compared to the 500 pounds he was adding). Definitely a good, fun, tall wake, with not much weight over stock. Looking forward to trying it with 750's, although I suspect we will have to also add weight to the nose at that point. Do think that the extra weight made the exhaust issue worse, and he is looking into FAE/downturns.
My overall conclusion is that if you want a boat with a good surf wake without having to add much if any ballast beyond stock, the G23 with NSS won't disappoint.