The bottom line is your SLA. If you are not 100% Outlook cached mode, blips in latency (above 20ms) for the Exchange DB or (above 5-10ms) transaction logs will cause end users to have to wait (OWA/mobile), or cause an Outlook popup (online mode).
To prevent this, you want to understand your SQL IO, and size appropriately. (Don't run huge SQL batch jobs in the middle of the work day, etc).
FlexShare can be used to prioritize the Exchange volumes over the SQL ones.