I've landed steelhead on 4 and 5 wt speys. If fish are smaller, half pounders especially, they are a lot of fun. And fighting a big fish off the reel is possible, but I find having the lifting strength to lift a larger fish at the end of the fight isn't there, and at that point I really want that fish let loose, it draws things out. I prefer a 6 or higher when expecting fish of 6 lbs or more.
Having a net person ready makes a difference, as it's fairly simple to fight them off the reel of a lighter set up, and have then fairly well subdued and hanging just out of reach of tailing.