I hear ya! Thanks for the extra background on this. Personally, I'd stick with WHM, but if the client is set on moving they'll have some challenges ahead of them. If you're not familiar with Plesk and you're looking to compile extra modules into the server, lets hope that your host has a pretty responsive team that can give you the support needed.
I did some digging around last night on google just to see what I coudl find on Plesk, and it appears that most of it is still command line driven, from an Admin's perspective.
We only offer the one control panel on our shared servers - cPanel/WHM. The main reason is familiarity, and the second being support. It's must easier to train staff on a single system, than have them jumble up 2 or 3 control panels and have to remember which is which
Fingers crossed for you. We do some custom compiles of Apache from the command line, but everything runs through the easyapache program - moving outside of the box I'd only be guessing as to what is where etc.
Hopefully some of the other hosts here who operate Plesk can jump in and give you some extra information.