I hate to disappoint you but as far as I've been told just a few minutes ago, there never has been a 993 TT Walter Röhrl Edition officially from Porsche.
This could be very true because as far as I remember, WR never worked for Porsche during the 993 production period.
The Walter Röhrl signature seems to be the same as Steffi Graf's signature...just a marketing gag from Exclusive department.

On our last visit, we saw a Steffi Graf signed Turbo but there is no special edition and the car didn't belong to Steffi Graf. Anyway: Agassi should consider himself lucky, Steffi Graf loves Porsche, she owned several ones, including the 993 Turbo S and she loves fast cars. Perfect wife, ehh?