Transitioning from rsync to ZFS send/receive

It sounds like you've tried all of the best compression mechanisms and are still being limited by the line speed. Assuming running a faster line is out of the question, have you considered just running the backups less frequently so that they have more time to run?

Short of that, is there some kind of way to lower the amount of data being written? Without knowing your application stack its hard to say how, but just doing things like making sure apps are overwriting existing files instead of creating new ones might help. And making sure you arent saving backups of temp/cache files that you wont need.

1. If you can transfer a maximum of 6GB per day (assuming zero overhead and zero competing traffic) and you are needing to move "15-60 gigs" at a frequency of "once or twice per week," that works out to 15-120 GB per week, or anywhere from 2-17 GB per day. Because it is necessary to plan for peak demand, and 17 GB is far in excess of even your theoretical maximum of 6 GB, it is likely that you have a very serious bandwidth problem. What will it take to upgrade the connection? If upgrading the connection is impossible, please consider the option of mailing physical media on a scheduled basis (e.g. weekly).

2. Assuming that you can get the bandwidth math to make a little bit more sense, rsync is likely to be the best option. Deduplication awareness would be hugely valuable when replicating highly redundant data (e.g. virtual machine images), but it should have little or no benefit when it comes to unique digital content (audio, video, photos)... unless, of course, users are inadvertently storing duplicate copies of identical files.