If you're going to be hosting multiple domains... why not just get your own dedicated server? That way you can install whatever software you want?
CandidHosting.com is reasonably priced -- and from my experience, the quality of the bandwith they provide is top notch.
If you're on a tight budget... try
RackShack.net. I know several people who host there.... and they're all happy.
As a bonus for rackshack -- I'm pretty sure their plesk servers comes with zend optimizer pre-installed.. not sure about the ensim though.
Either way -- installing the zend optimizer is a breeze... I've done it on my own personal servers without any problems.
Good Luck!