Google Apps Sync
It looks like you are so close to supporting the Google Apps Sync connectivity method but it still doesn't seem to work. My employers has IMAP and SMTP/POP disabled and uses only Google Apps Sync or the gmail interface. This would be a HUGE improvement and I'd happily donate to the development costs associated with it!
Marty Durkas commented
Yes, the fact that the "Mark as Spam" feature only applies to mail in the context of Airmail makes it pretty worthless. Besides even if all I use is Airmail for email and never use the web client, it doesn't seem to do any sort of learning that Google does to capture future mail as spam.