When I had my web site up and running and did more with the Walther line of handguns than I do now, which my site favored, I received this directly from Walther GmbH. It is dated 12/27/02.
Reader's Digest version...
"It is possible that a P99 chambered in .45 ACP will be built in the near future, we discussed this point already, but we do not know exactly when this will be, maybe next year but not yet sure."
I have kind of lost interest in Walther handguns so I've had no contact with them recently but, the info that I was getting on a regular basis was very reliable.
Personally, I got the impression that they were quite satisfied with their part in the SW99 in .45 so...???