I'm basing the "good returns" on my catching anywhere from 4-12 fish an outing, not only on the Stamp, but a number of others rivers as well (which I obviously won't be posting here). Obviously some rivers will not be doing well or less well than others. As I said, the Gold hasn't been doing that great, which to me is surprising considering what I've seen and heard elsewhere. I'm not doubting your experience on the Quinsam - I was focusing more on the "other island rivers" mentioned in your post (rivers like the Quinsam IMO are toast - urbanized, sedimentation, pollution, etc).
Buddies up North island, and down island as well are reporting the same as I've experienced. Great numbers of fish, most of them are getting 2-4, up to 11 fish per afternoon outing they are doing. Probably not as good as the "good ole days" back in the 80's, but it sure as heck is a positive outlook compared to the last couple of years we've had. The fish are way larger than average this year too, we've hit quite a few now in the 14-18 range.
Hopefully the up trend continues, last season was a real downer.