I currently purchased a dedicated server hosted at iWeb and got it administered by them.
I recently asked after registration to add php_apc and php_imagick to the available libraries. It seems according to them that it is not possible as it is not supported with cPanel.
I would apparently need to do that myself... is there any risks to install those two libraries ? What kind of problem could it raise if there is any ? In case I would have to debug this myself.
Installing Imagemagick to your system in itself won't be too much of a problem.
However, adding support for it and APC to PHP may be a tricky procedure; it may be best for you to just no longer use the PHP provided by cPanel and install PHP yourself, which will go into /usr/local, and run the configure script for it yourself, compiling in whatever extensions you need. It'll mean that you'll need to keep PHP up to date yourself, but it'll also mean that you won't have all your customisations to it wiped the next time cPanel upgrades it.
If there are better suggestions I'd also be interested to hear them.