We'll automatically prompt you to compact when we see a reduction of a certain percentage in the number of messages, etc. We'll actually call the same code as when in the Exchange world, you say File->Data Management->Settings->Compact Now.
.zdb files keep growing, even when you delete things. As far as you're concerned the messages are gone, but the space is never reclaimed. You will notice that even if you delete and empty trash, .zdb files keep growing. Outlook will perform much better if you periodically compact the file. It can take awhile on a big file (maybe 20 minutes). So as far as your question about performance degradation after compacting, I would think it is the other way around -- performance will degrade if you don't.
Another alternative, of course, is to create a new profile and initial sync. You will notice that if you do that, even though you would have the same messages in the new .zdb it would be a lot smaller.
There are many postings in the web about data compaction in Outlook. [You can search for Outlook, Data Management Settings, Compact, etc.] That can give you some more info.  | 
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