Since no one responded...
1. Hobo just adds function, doesn't replace or remove. I.e. you can
still use erb. I have not run into any problems with plugins other
than trying to get them into dryml (but it's always an option to use
erb). No particular experience with payment plugins
2. Hobos javascript is based off prototype (like rails). Shouldn't be
too hard to use another, but removing prototype calls from hobo code
would probably be somewhat difficult and just adding another
javascript framework on top can be a significant bump in download
size. Though if you don't use most of hobo's automatic ajax stuff and
write your own, it wouldn't be a problem.
3. I've gotten hobo working on mod_rails on an ubuntu 8 box.
Shouldn't be any harder on debian.
4. Unknown by me, though I've seen a fair hit in speed for what little
code I've done in both dryml and erb. Like 2 or 3 times slower but
might've been fixable with right optimization.