Or maybe depending on how much the clients are paying, get each on their on 360

, though availability is pretty low at this point in time:
http://www.linode.com/avail.cfm
What do they need/want cPanel or anything like that for? Perhaps you can find other alternatives.
email - use Google
DNS (sub-domains, etc), if each had their own Linode, they could take care of that it in the Linode manager. I think you could use Linode's API to do pretty much anything you need there so you wouldn't need to give them access to the LM.
File manager? There are others out there you could set them up with.
One-click installs of software? Screw that. Nobody keeps that stuff up to date. Instruct/guide your clients on manually installing WordpPress, Drupal, etc.
Just a couple of examples.
You could (I think) get a separate Linode and setup cPanel or some other control panel on a dedicated node. So those resource pigs aren't consuming the money maker box
Looks like cPanel has a VPS version, but it still appears pretty dang hungry:
http://www.cpanel.net/products/cpanelwh ... ments.html
Seems to me cPanel has only had their programmers work on polishing the front end interface and can't figure out how to code the back end for efficiency. Yes, it does a lot, but I don't understand why it needs so many resources. In my mind it should really be a polished front end to some scripts that do work. It should be pretty dang light weight.
http://www.cpanel.net/products/cpanelwh ... mized.html
Lots of options though. I've used hosts that use cPanel and those that have home-grown solutions. I've always preferred the home-grown solutions over cPanel as an end-user.