I believe that Walther P99 sights are compatible with the SW99, based on the fact that while shopping for Night Sights for SW40VE last year, I had a lot of SW99 sights show up in the search results, and nearly all of them were listed as "P99/SW99 sights".
Seeing as the SW99 was a licensed variant of the P99 that was a joint venture between Walther and Smith & Wesson, it would be sort of weird if they had decided to give it unique sights.