We've built a tool (which we call Compass) that integrates Kentico to iMIS covering authentication and access control amongst other things (renewals, events, training, cpd/ceu etc).
Rather than integrating by copying contact data into Kentico we maintain iMIS as the source of the truth for profile information and do real-time integration as required. We've extended the authentication process in Kentico to authenticate against iMIS (through Compass) and then return a collection of roles back to Kentico to use for controlling which members can access which pages.
The roles are dynamic though - so as an example you could set up a rule that says "Put someone in the Member role only if their paid thru date is in the future and they're one of these three member types."
Compass is a product we've built which sits on top of the iMIS API so if fully upgradable also. Let me know if you need more information.