Gregor Paul: Sorry Sonny Bill, you're no Brad Thorn

Sonny Bill Williams wants to be the world's greatest cross-code athlete. Sorry mate, but that honour will probably always belong to Brad Thorn.

Even if Williams becomes heavyweight champion of the world - possibly a ludicrous notion, but possibly not - he won't have a CV that holds a candle to Thorn's.

Frankly, it's ridiculous what the 37-year-old has achieved. He is the most successful league to rugby convert of all. The fact he made the switch twice, is mind-scrambling.

There have been some league converts who have shaken up rugby for a while. Jason Robinson was a livewire for England and the British Lions and won a World Cup medal in 2003. Lote Tuqiri had his moments with the Wallabies but was just too high maintenance; too much of a goose off the field to ever really conquer the 15-man code. Alan Tait was a trooper for Widnes, Leeds and Great Britain and then played a key role for the British Lions in 1997.

These blokes were good league then good rugby players -just not quite as good as Thorn. His legacy in both codes is enormous and he collected a final trinket at the weekend which gives him a set of medals no one else is ever likely to match. Drafted to Leinster on a short-term contract, he helped them win the Heineken Cup, to add to his Super Rugby title of 2008 and World Cup medal won last year. Between 2008 and 2011 - Thorn was one of, if not the best tight lock in world rugby. Ask anyone in Brisbane about Thorn and they will go misty-eyed and rate him one of the best ever produced by the Broncos. A master of two codes.

Williams, for all his potential, for all his athletic gifts, hasn't dominated either code in the same way. His time in league was more a dalliance than anything else. An NRL title-winner with the Bulldogs at 19, he didn't hang around long enough to make the same impact as Thorn. He was more about the potential than the actual.

He's now looking like a seriously good rugby player since he joined the Chiefs. They use him well - he hits the ball hard, looks to cross the gainline and has learned the art of knowing when and when not to offload. He's been the best second five in Super Rugby by some distance and could take command of the All Black jersey this year - certainly in June while Ma'a Nonu is given time to come right. Williams has the World Cup medal to sit alongside the NRL silverware, but he's still short of Thorn's haul.

It is more than just medals that sets Thorn apart, though. Longevity - consistency of performance across a 20-year playing career is impossible for Williams to compete with.

Time is on Williams' side but only if he commits to rugby for a few years more. If he returns to league next year, his rugby legacy will be well short of Thorn's.

If he hangs around until 2015, wins a couple of Super Rugby titles and a second World Cup then he'd be a challenger to Thorn. He could then return to league, win a few more boxing bouts and possibly then he'd have a claim. But only then and right now, it is hard to see Thorn being surpassed as the world's greatest cross code athlete.