Vinnie,
I even use the split thread technique with 6/0 and 8/0 thread because it is quicker and easier than forming a dubbing loop, advancing the thread to the front of the fly, putting in the dubbing, twisting the dubbing loop, and then wrapping the dubbing loop. Far easier to simply split the tying thread (or floss), insert the dubbing, spin the bobbing, and wrap the body.
Using the split thread (or floss) technique has less bulk and forms a beautiful body.
Everyone,
Whether you use the split thread (or floss) technique, or use a traditional dubbing loop you can vary the thickness or slimness of the body very easily. If you want a very slim body, when you put the dubbing in the loop, simply twist the dubbing onto one of the threads (or sides of the split thread if using split thread technique) before twisting the loop (or spinning the bobbin in split thread technique). The result will be a very tight and slim dubbed body. If you want a fuller body, put the dubbing in the loop running parallel with the thread and then spin the dubbing loop (or bobbin). For a very full body, put the dubbing at right angles to the thread and then spin the dubbing loop (or bobbin).
Glasso used to put an underbody of flat silver tinsel on his bright bodied speys (like the Orange Heron and Sol Duc Spey). This tinsel underbody keeps the floss from loosing its brilliance when wet. After he made the rear 1/2 of his body with a double layer of flourescent floss, the split the floss, put the first 1/3 of the dubbing in the resulting loop and twisted it to one side of the floss strand before twisting the dubbing loop tight(remember this makes a very slim body). By doing this, Glasso was able to make the transition from floss to dubbing very smooth without having a bump. He put the dubbing for the rest of the dubbed front body half in the dubbing loop parallel to the split floss and twisted it.
How do I know this some may ask, simple, I had Dick Wentworth and Don Kaas show me. Dick was taught how to tie flies by Glasso when Glasso was his school principal and was his fishing buddy for many years. Don was taught how to tie spey flies by Syd when in his early 30's.