Sickclown,
When I first read your question I was going to answer that to steal the 8 weight would just be wrong. Now I see you're asking something else, but the something else isn't entirely clear.
". . . his 5 weight does better in his pulling force tests then the 8 weight that he was using." I haven't read the book, so I have to ask, what does the author mean? I haven't heard of "pulling force tests" as applied to fly rods. Fly rods used for saltwater fishing are measured by their "lifting power." That is the amount of dead weight a given rod can lift off the ground with the rod grip held level. In that test, I think few, if any, 5 wts can dead lift more weight than 8 wt rods.
Like most anglers, I don't lift steelhead vertically with my rod. I pull sideways toward the beach. Still, any of my 8 wt rods will pull more weight with the rod bowed than will any of my 5 wts without breaking.
Without more information about Mr. Scheck's pulling force test, the reported results simply don't make sense.
The lightest line wt rod I've ever caught steelhead with was an old Scientific Anglers 6 wt fiberglass rod. I was under-gunned in that instance, not that the rod couldn't do the job, just that it couldn't do so comfortably nor quickly. The lightest graphite single hand rod I use for steelhead is a 7 wt. However, it's a medium action rod, and I've handled 5 and 6 wt graphites from manufacturers who build rods stiff as fireplace pokers. Those rods likely have the same lifting power as my medium action 7 wt (and many of those stiff rods comfortable cast lines one or two sizes heavier than their ratings). Somewhere in that range appears to be the lower end of rod wts that steelheaders generally find comfortable for steelhead fishing.
If you prefer fishing your 5 wt over your 8 wt, give it a try and see if it's the best adapted solution for the lines, flies, water, and steelhead you fish for.
If it weren't for two handed rods, I'd stick with my 8 wt for winter fishing and 7 wt for floating line summer fishing.
Sincerely,
Salmo g.