I find there are no "generalities" when it comes to which flyline works best for a particular rod. I have an 8'6" 5-wt, moderate-action rod that I was happy with using a 5-wt line. When I tried a 6-wt line, the rod simply came alive and I've never used the 5-wt line on it again. I can also cast weighted, beadhead wooly buggers up to 60 feet on this set-up using a double haul, and at the usual shorter distances, it casts like a dream. This is not a high dollar rod either.
Conversely, I have a fast action, 6-wt that I bought at LRO last year. Before purchasing the rod, Daniel assisted me as we both tried it out on the grass. I asked if we could cast the rod with a 6-wt line, plus a 7-wt. I brought along a bulky, weighted Matuka Scuplin fly to test the rod with. This fast action rod cast beautifully with the 6-wt, but felt dead with the 7-wt line... like casting a cable.
Currently, I use two lines for that particular rod, both 6-wt (nymph taper, plus a fully compensated, full sinking line) and it casts beautifully with both.
Therefore, I think you need to try before you buy. Each rod is different and you should not make assumptions about how a rod will cast with a particular line. Take your rod to the shop and ask for their help before buying a new line. You might be surprised... hopefully, you will be pleasantly surprised.
Remember, a fly outfit that casts well must be properly balanced. It's not guesswork... you gotta try things out.
Tight loops to you... Scott