Over 100m, I'd say that Tahu (in his prime) and Mullen are the same.
However, the fastest player I've ever seen since following league would have to be Darren Albert (when he was at the Knights, circa 1996-2001). Now, he was VERY FAST, and use to win the 100m challenges they use to hold on the Footy Show all the time. I think his PB was slightly over 10 seconds flat (10.3s I think). Even when his best years of football were behind him at St Helens and Cronulla, he was still pretty quick. Some have said he's the fastest player in the history of Rugby League.
The person who comes in at a close second is Martin Offiah from the UK, during the late 80s and early 90s. I only saw Offiah play towards the end of his career in the mid-late 90s, but I can see why they use to nickname him "Chariots", from looking at old footage of him