Well, I spent quite a bit of time with the XLT Bruce was kind enough to send along.
Although you can not consider this an expert report from a Simon, Dana, Tyler, etc - you can consider it a report from a fellow thrasher on the water FWIW
Here's my report: (short version, will do more extensive in an article later)
Line: SA XLT 9/10 marked "Experimental", bright orange coating
Leader: 12 foot 60/20/20 recipe with yarn
Rod: Sage 10150-4 Brown, 10wt 15foot 4-pc Traditional Spey
Single Spey, both sides
Double Spey, both sides
Reverse Single, Reverse Double right side only
Reverse Snake Roll
Although I am far from 'expert' in all of the list above and don't use some very often, I was very pleased to be able to cast the line fairly well with most casts provided I was able to throw enough of a kick into that l-o-n-g taper to load the rod properly into the d-loop's vee and make sure that much line was kept 180 to the forward cast, or vice-versa. That was the key to taming the huge taper, not unlike a double taper but it doesn't go flat after you hit the limit.
One standout characteristic of the extended belly is the way it loves the snake roll, which really makes casting a long line quite easy once you find the right place to "flip the egg" as Simon himself puts it. I haven't been doing the snake roll as long as say Dana or Tyler or Double Spey but I felt like I have with this line. The above comments pertaining to big sharp dee loop apply.
I found it difficult to single spey such a huge, long belly unless I reeled up to shorten it - probably a shortcoming on my casting ability. I was unable to manage the whole taper where I was standing fairly deep to avoid trees, which were a problem with the length of the dee loop needed by such lines. That being said, there was an optimum line amount for single spey casting and I was surprised that when there was enough line speed the fattest part of the belly actually shot thru the guides albeit with some effort and not like running line of course. That probably speaks to the slickness of the coating as well, which is a brilliant orange. I would like to see this line in their green color like the old Steelhead taper lines.
When the whole head was out, most of the casts I attempted were beyond my skills with the 10150-4 wading a bit too deep to avoid the trees. After spending some time trying I am even more impressed with Steve Choate and anyone whom I saw do the whole 9/10 belly in a single spey at the Spring clave.
Except for the snake roll with a very deep kick into the dee loop, many of my casts felt heavy and it was hard for me to keep the line stick from killing my cast. I felt much more comfortable reeling up a little to get back to a favorable length, when nirvana would return. So for my skill level, all that extra line would not do a heck of a lot for me unless I gradually worked up to that or perhaps found another rod that could work it better(?) I've been so happy with the 10150-4 sage that I tend to think in terms of a line for that rod instead of a rod for the line, but I could be wrong.
Shortline casting was definitely do-able with that sweet taper, although after casting the big belly it felt like there was no load on the rod the loop formed easily and it will work for tight quarters when needed.
Best casts with the combo for me: Snake Roll, double-spey(partial belly) reverse as well, single spey (partial belly)
Worst casts with the combo: Snap-T either way, left handed reverse anything (but that's my fault)
That struck me as a bit funny because my usual best cast is the Snap-T when using shorter belly lines. I think the super-long belly makes the sweep more difficult due to the increased surface tension of the amount of line in the water and the line stick kills me.
I think this is a great line for big river applications for a full dry line. I've heard that they will work if cut for tips, but I noticed that the last part of the taper is quite light and would have to see the line in action with a type IV before deciding whether it would suit my Skagit River methods or not. It will be fun finding out though!
I will not venture to compare to other lines until I try them in the same context, but would prefer to just say what was great about them as well as where I had difficulties given the limits of my skills.
In summary, the biggest plus for me was that this line made me feel like a snake roll machine. The biggest minus for me was that the whole belly seemed reserved for casters with a big "S" on their chest - a lot of hard work at that length, which is the not what Spey casting is to me. I'll reel up and get back to that sweet spot somewhere on that big continuous taper like I always did with the DT, but it's nice to know there's more where that came from for that day I shake off this kryptonite shackle.
Of course when everyone else tells me how easy it was to cast the whole taper I'll be the goat
My utmost thanks to Bruce for the opportunity to try this line, and hope I get to try other lines in the future. I believe Sinktip is next in line for trying this line... Duggan please PM me your mailing address and I will get it to you.