Not only have I read all his works, but Wittgenstein is one of my favorite philosophers (second only to Camus), and I'm actually on my second read through of Markson's Wittgenstein's Mistress right now!

I think that Wittgenstein's ideas don't offer as clear a view on animal sentience as people seem to think -- there's a clear tendency to regard his direct quotes on animal thought as his way of communicating his stance, completely disregarding the fact that the man was notoriously cryptic even beyond his stylized, devil-may-care approach to philosophical writing. In my mind, the substance of his ideas suggests a much more sensible (and arguably modern) take on the issue, in that he wasn't arguing on matters of sentience so much as he was attempting to cultivate some perspective on the alien nature of minds fundamentally different from ours.

That said, my opinion becomes a bit more untenable when it comes to the pain argument you mentioned. The basic idea could still be applied, but it's not quite compatible with some of the abstractions we see in his writings on pain. Truth be told, I don't think Wittgenstein's thoughts on pain behavior in animals are entirely original or significant, and while I can appreciate his dismantling of the duality inherent to any sentient being, I also think that science is better equipped to address this idea than philosophy (though the former might well be informed by the latter in this regard). I think that he and his are on firmer ground with regards to the PLA, which science will have a much harder time addressing.