Not all 6wt's are built the same...fished a 12' 6" NRX 6wt the other day on the Chilliwack/Vedder River and landed a 13-15lbs steelhead. River was very low with little current. I also have a 11' 6" 6wt Rainshadow Immortal but won't fish it for steelhead as it will bottom out.
It also depends a lot on how you fight a fish - with the rod (i.e. with the butt or mid section), the type of reel you use (i.e. click and pawl vs. drag) and how you use the two together.
A 6wt spey should be fine for eastern tribs...I fished a 7/8wt NRX on the Saugeen and it was overkill.