Re: How is Rio Grande different than just over lining?
Much data is available from RIO and others on the weight of the first 30' and total head weight. Along with this and at least as important, is the taper diagram illustrating where mass is located in the lines design. It is a myth, at least partially so, that faster action more powerful rods need over lining. Some do, some don't, this is a rod design issue. A line like Grande is not simply nearly a line size heavy, it is also tapered for aggressive deliveries; larger, heavier or wind resistant bulky flies. It lacks the smooth stable charm of Gold with its longer front taper and extended rear taper. Long rear tapers not only facilitate reaching and mending and eliminate hinging but they also promote long cast loop stability after the whole head is out the tip-top. Gold, SA Expert Distance, Cortland BOSS and a new Airflo line all incorporate this 20 - 25' rear taper (initially developed by Bruce Richards at SA).
Knowing the grain weight of a lines 30' and total head is good but you still need to cast different lines on a given rod to determine what it likes best in your hands. Sometimes differences are subtle, with other rods, extreme. Some rods are easy: along with 3 expert casting friends, we lawn cast my beloved Zenith #5 with 3 lines prior to Hendricksons starting. It cast all 3 just fine and we had to repeat the process 3 times to give one line the edge and I am still uncertain we were right (I've been fishing RIO Gold on it but we decided Cortland's new Trout BOSS might have a slight edge...I did bring a fine brown to net with it later in the afternoon). Other rods, my favorite bonefish rod the Loomis NRX #8 comes to mind, are more line temperamental. I tried lots of #8, 8.5 and even 9 lines on it before finding two that sang a sweet song; RIO's #8.5 Tarpon and even better, SA's #8 Textured Saltwater...lighter but a more aggressive taper.
There is plenty of science in rod-line mating but even more feeling. The problem is getting a broad assortment of lines to comparatively experiment with. Employ your friends as a resource; switch like sized lines among your different 5-weight (or whatever) rods and feel the difference. Oh, and it wold be great if line makers would make their best designs in 1/2 size increments.