Hamish / Dave,
I agree totally with the above, it does depend on the angler and their abilities.
This time of year I am fishing a sinking line and a large heavy fly. I never overhead cast with a heavy Cu or W tube, even on a DH rod, I do not feel comfortable and would not like to do it on a singlehander.
When I talk of salmon fishing with a SH rod, I am talking about mid season fishing where one may be using a size 8 or smaller fly and fishing a floating line with a 5 or 7 foot sink tip, possibly a full intermediate. When considering rod and line weights, we need to think about the function of the line relative to the task, it needs to be heavier to carry a heavy fly hence a WF5 will not turn over a 1.5" Cu tube, to turn a fly of this size needs a line of quite high mass, it gets to a point where the weight of the line is too much (for most) to cast with one hand so we use two and a DH rod. This is not the only advantage, you have the ability to lift and manipulate more line with a longer rod.
Single handed outfits for salmon fishing need to be properly balanced particularly if the intention is to Speycast with them, there are lines out there that will make it a breeze but it needs some thought. I tend to use a heavy short headed line to Spey cast with on a SH rod, currently favour a Loop Opti Stillwater, rolls out a 7IPS sink tip and will shoot a huge distance from a short carry and small D loop. I will do a demo on single handed salmon techniques at the Orvis / ADAA day on the Dee in April, with the right set up it is possible to chuck 1" Cu tubes and heavy tips on a singlehanded rod.
In terms of rod power and ability to land fish, 9 foot rods rated for 8wt lines are often used and designed to be used for baby Tarpon and salt species up to 30lb so I see no reason why, in the hands of a competant angler, a stiff 8wt reservoir rod cannot be effectively used to humanely land a 15lb salmon. I have landed spring fish up to 10lb on a 9 foot 6wt rod in under 15 mins, the fish have all swam off strongly. In some cases I probably would not have hooked them in the first place had I been fishing a DH rod with a heavy Spey line.
Dave23, I would not like to say "yes, go do it" without seeing the outfit you are talking about, if the rod is a soft 7/8 and the line has a fine tip, long front taper and long head then it could be nothing less than painful!
Jim, spot on.
I fished a small river near Edinburgh when I was at Uni there that often required heavy tips & large flies but you could not get near it with anything more than a 10 foot rod, anything more than a 7wt is uncomfortable for me to Spey cast for extended periods and a good fish there was 7lb, I have had 5 & 6lb wild brownies on 3 & 4 wt rods so a 7lb salmon on a 7wt is perfectly controlable IMHO.
Cheers
Ben