Hello again Gus,
I spent some time casting the Rio Windcutter 9/10/11 on the Expert 1409 this morning. I had planned to compare the 8/9/10 and 8/9 Midspey but I got paged to less pleasant duties.
I tested the (54' head) Windcutter in case you are fishing smaller streams, and/or if sinktips are a consideration. Also, if the retrieve includes stripping after the swing, the shorter heads rule. The Windcutter also turns the fly over very well when only part of the head is beyond the tip, which would help for short lining in tight quarters.
Again I don't know what your situation is but nonetheless I can tell you what I found out FWIW...
The Windcutter 9/10/11 w/ tips is a gun on the Custom 1409SP, and it also works very well on the Expert 1409 provided you like a fully loaded feel when casting. Considering Simon Gawesworth's two-level line ratings, the 9/10/11 would be on the fuller load side, and the 8/9/10 on the crisper, lighter side. So it really does depend on what you like.
There was no lack of muster in the casting, the rod has the kick for this line. To lessen the weighty feel, I stripped the back taper into the guides a bit and it shot just as much running line.
Just as I was going to spool up the 8/9/10, I got called off the water. I can imagine that it will be the lighter, crisper sister as Simon lists on the Rio site. They'll both work, preferences are the key here.
If your fishing situation calls for a longer spey head - based on grain weight ratings and the comments of MJC and sinktip, I would venture that the Long Delta 9/10 is an ideal match at 690 grains over 66ft. My guess is that the Midspey is going to feel a little heavier than the Long Delta, so I'd go with two guys who have firsthand experience.
SUMMARY:
If you plan to fish (a) smaller rivers, (b) use sinktips a lot, and (c) like a distinctly loaded feel the Windcutter 9/10/11 is a good choice. If (a) and (b) but you prefer a lighter crisper feel, go with the 8/9/10 Windcutter.
If your fishing situations work best with a mid-length belly spey line, then go for the Airflo Long Delta 9/10. I'm confident that it's a great match for the rod and the right mid-length line for it.
If you have any trouble locating either of these lines I think Mike or I can help you out!
Juro