This is a very interesting mod because of the differences between the NS999ES and the NS9100ES. The latter has a separate, rather than shared, power supply for the digital processing card. Not only that, but it's a linear supply, not a switch mode power supply. The 9100 also uses a separate clock for audio replay - it does not share the DVD video clock like almost all other universal players. I'm not sure how much that counts in the big scheme of things, but I'd expect it to unlock a significant improvement in sound quality. I'd be very interested to hear what people have to say about this one.
Nick