Stuart's 25 and been around a while now. He ought to have the scrummaging pretty much mastered, although some good specific coaching will help him improve further. The bigger issue is probably getting out of a culture of mediocrity and under achievement. I'm sure he'd quickly look a heap better somewhere like the current Tigers team or Sarries.
In reality 6'6 is the minimum for an international lock. Kruis balanced hefty with athletic in the line out, but where we've really missed him is his style of play - seldom spectacular, but consistently just throwing his weight around doing the dirty, grunt work that enables the piano players to play. Personally I'd prefer both locks to be from that mould, but other views are available.
I've always been a big Kruis fan and reading these boards he's definitely got better by his absence! But if he returns we shouldn't automatically expect the Kruis of old. He's 32 next month, isn't battle hardened from a top league and I think I read that he'd lost some weight to help with the quicker Japanese game. On the plus side he'll have been playing less rugby so will have been able to rest and look after injuries better. Will be interesting to see how long it takes him to get up to international speed.