Before you cut that line, here's my experience with it.
Last year I bought a WC 6/7/8 for my 6126-3. The line cast like crap. Thinking it might just be my modest skill level, I checked with Amy Hazel, who uses that rod a lot for her classes and guide business. She agreed with me; that line is at best a mediocre match for the 6126.
I ended up with two lines that work very well with that rod: A Vision Ace shooting head, and a Delta 6/7. Both cast WAY better for me than the Rio.
I use the 9-10-11 interchangable head on the 6120 without the middle section. Total head lenghts between 42 to 45 ft are my sweet spot. have no problem flippin' the RIO provided floater and
Type III sinkers with this setup. The Type VI 15ft. tip and a big bug can be a bit of a pain.
As Dec said, "15 feet". All the RIO spey lines get cut at 15' for tips. I disagree with those who think the Wincutter 6/7/8 doesn't work well on that rod, it is a very good match and it weighs nearly the same grs as the 6/7 Delta Spey. The rod doesn't care what line is used, it is designed to cast a given window of grain weight.
You're right, the WC and Delta are very similar. However, I cast them side-by-side to test whether they cast the same on that rod, and they don't.
With the Delta I was able to get nice tight loops and good distance. The WC would not cast tight loops regardless of how I modified my casting stroke. I can't explain it, and going into the comparison test I didn't expect to see any difference, but nevertheless it was very apparent those two lines don't cast the same on that rod. If you haven't cast them side-by-side you ought to try it. The line specs obviously don't tell the whole story on how they'll perform.
I am curious as to why different lines are seen as better than others as Mike T says. You read/hear opinions like that a lot. I am pretty new to spey casting but it seems to my simple brain that if you blindfolded your rod and made a cast and then asked the rod what line it was it probably wouldn't know for sure. It just gets loaded with what is there. I cast a Delta along side my WC and the Delta was better but I would guess that was because it was a size up (and recommend by the AATF folks to load my rod correctly). My casting abilities are so newbie I couldn't really pick up on the nuances of the two lines.
It seems that once you chop your 15' you ability to tweak the load on the rod is only restricted by your casting ability and your the money in your wallet.
I have not tired nymphing with the Skagit line. I would think that it would work. I do have a modified floater that I got from Red Shed. It works well for an all around line. It Will throw big flies no problem, which is one reason it was created. Look at the WC 678 w/ tips. That would be the all around line.