Of course playing in the NRL improves players, IMO.
You need to play at a higher level week in week out and you come up against higher quality players week in week out, i don't think theres any question.
Our best players yesterday burgess graham widdop (amongst others) play in the nrl. Sure they were good before. Sure they would have been good in SL. But they wouldn't have been able to maintain that quality for the full 80 IMO.
Frankly, the more players we get in that competition, the better england will be. Get charnley, hall, watkins matty smith and we'll quickly start to have a team mainly made up of players that play at that level week in week out.
It's wierd that, as the super league competition itself becomes drained of stars, is when england could finally achieve success.