The 8/9 MidSpey works fine on the rod; however, the rod as currently designed does not have the casting ability of the 1409-3 or 1409-5. And this is due to its softer mid-section, which is what I would like to see redigned in the same way that the 1309-3 was redesigned.
The 1509 as currently made does not like casting the GrandSpey (either the 8/9 or the 7/8), nor does it like casting the 7/8 or 8/9 XLT, and it should be able to do so easily, just like the 1409 does. The softer (weaker) mid-section is the limiting factor in this.
Yes, there are many who feels that the 1509 as currently made is more fun to fight fish on than the 1409-3 or 1409-5, and this is because the softer mid-section causes the rod to bend further when fighting a fish. The trade off, is you lose some casting ability and the rod has a tendency to fold a little when you push it for the really long casts (100ft or more). I'm also aware that the 1509 as currently made feels more friendly to those who have been fishing with slow or medium slow rods because the softer mid-section slows its casting recovery down a bit.
I'd much rather have the more powerful, faster mid-section (like the 1409 and 1309 since its redesign) simply because it casts better. I want to use this rod for larger rivers like the Sky, Hoh, Cowlitz, Skagit during summer/fall low-flow for fish in the 5 to 15 pound area with small low-water flies and wakers. This means I want a rod that very comfortably casts the long-belly lines 100+ feet.