What is it you don't like about the forks? It the internals that do the work, why replace the tubes?
You can do a full swap (triples?, forks, brakes, wheel) to the 636 or zx6r... just do a search on it.
Personally, I did Racetech springs and Racetech gold valve emulators. Although they don't allow adjustments as easily (it's all internal)... they seem to be miles better than factory. I only have about 200 miles on them so far.
You'd have to do a cost comparison to see which you prefer. I got the springs/emulators for $200 total (unused with a bike purchase), then had them installed while having the forks rebuilt (which they probably needed being at 24k miles). My total out of pocket was $500 including all seals, oil, labor, springs, & emulators.
The 636/zx6 stuff you'd have to find used, and then decide how much needed rebuild/replace. And figure out something with the handle bars. So, there isn't really a direct comparison between the two.
BTW, what they say is true... after doing the forks, you just want to spend the money on the rear shock. Front is nice, back feels like it kicks/bucks now.