Hi @fulltilt
interesting Application and good to have a community version of this opensource solution. In coming version (full rewrite!) it wants to be responsive. Roadmap for 2018 tells us improvements on version 6.x.
Well, there are a lot of good webbased applications out there. Because its focus on security, privacy and federation I prefer nextcloud with may be far more options and features. But you are talking of an integration into i-mscp. Currently we have rainloop and roundcube as webmail solutions. Both with their advantages, well tested and sufficient as a fallback, if imap for any reasons fails.
The developers team of i-mscp is very small. So I would be glad, if they can concentrate on the core of i-mscp. Lets have reliable and responsive software here. Or have an upgrade feature from within. Or present more system information. Anything else related to webadministration. Oh, yes, the multiserver thing 
My suggestion for us as a community would be to fill a table telling us s. th. about the compatibility of these versions. Say Group-Office v6.3 is running on i-mscp 1.5.x without problems or some small tweaks or needs certain plugins.
What do you think?