This is exactly what I always say. Mason is very quick off the mark which gives the impression he is "fast"; That's great, but I'm yet to see him in open space or running someone down.
I guess some see it, some don't, but over 100m I'd say Bula would cover Mason pretty comfortably.
Relevance to league? Well, this comes in when put into a gap or chasing down a winger like Bula has done (e.g.; chased down Katao from Cronulla).
The thing with Bula is physical motion doesn't equal speed - it does give the impression of it though.
I haven't seen enough of him, so can't say for certain about Mason, so I guess not entirely fair, but I'd put Bula amongst the fastest at the club (Staines probably quickest?).