Considering 4.4.3 isn't even officially launched yet, and is rumored to launch sometime in May...that would indicate that Sony does not yet even HAVE 4.4.3 to work with.
So no, unless they've mastered time travel, I don't see how they could possibly release 4.4.3 in May. My guess is we'll never see 4.4.3 on the Tablet Z. However, the next major release is entirely possible. A 4.5 or 5.0 release is entirely possible around end of 3rd quarter from Google(maybe announced in late June at Google I/O, released a month or two later). Maybe they could have it ready 3 months of so after release. I wouldn't be surprised if this tablet sees at least one more major update after this 4.4.2 release in May. That following one may be the last though.