Long focal lengths is the one real shortfall of the double-extension Tachi (I have one, too). To go beyond about 450mm (with some room for closer-than-infinity focus), the only option is a true tele design, like the Nikkor Ts, etc. Unfortunately, they don't have much in the way of image circle. The Nikkor 800T, for example, appears to have an image circle of only 200mm - not nearly enough to cover 8x10 when focused at infinity. The flange focal distance of the 800T is about 527mm, meaning it would just work with 22" of bellows, but not fill the frame with sharp image. [edit, added] The 800T is supposed to weigh in at 56.4 ounces - not exactly light-weight.
At the close end of the spectrum, I've mounted my Nikkor 360T on my 8x10 Tachi double-extension, racked the bellows out for close to 1:1, and it appears to cover OK. I haven't done actual test negs to check real coverage and corner-to-corner sharpness, however.