One doesn't necessarily have to follow the Star Trek Encyclopedia's take on transwarp. It is far more likely, IMO, that "transwarp" is a catch-all term for "faster than current warp capability" ("transwarp drive" never functions the same way every time we see it, anyways) and that Excelsior was succesful, necessitating the warp rescaling before TNG (and explaining the shift in starship design from Excelsior onwards).
As for transwarp/slipstream scaling, the idea for "Transwarp 1, Transwarp 2, etc." works much much better than "Warp 9.991918212891" or whatever.
Anywho, judging from VOY, slipstream seems to be a pretty decent transwarp option, seeing as Voyager already did a fair amount of R & D on it. The main downside is that it seems unpredictible, unrefined, and difficult to control. I wouldn't reccomend casual use, unless it's seen decades of development into something I would feel safe using.
Just my two quatloos.