The 10-20 you have is the original style. It was made on a slow tapered mandrel and in my opinion was not a true Magnum taper. The new one is a bit more like your 8-15 a true mag-taper. That being said the first generation 89 was a great rod and very sensitive, just did not fit the series as well as the newer one.

Will 10-20 work for chinooks on the willamette & clackamas river or should I use a beefyer rod?

The 89 MT in all it's forms in the Lamiglas line has always been considered by all anglers I know to be a fine salmon rod.

10-20 lb. rods will handle any salmon or steelhead here in the portland area, including the Columbia!