• many dedup appliances will not only dedup but will do traditional compression.  Compress your SQL dump with your favorite compression utility.  Your dedup appliance is unlikely to do better at compression.
• If you run your SQL dump a bazillion times, you'll see an incredible dedup ratio.  All this means is that if it's important to backup the same thing over and over, your dedup appliance will do a nice job. Many "stupid" backup programs will backup the same thing over and over.  They can't help it; they were brought up that way.
  
• If you make a SQL dump of many different database objects, your dedup appliance will or may find identical blocks within the dumps, allowing you to store the dumps using less appliance space.  The dedup you get from these sorts of storage may or will increase somewhat as you fill your appliance with various stuff.
  
• Even within one SQL dump, your dedup appliance will or may find identical blocks, allowing you to store the dump using less appliance space.
• A dedup ratio of 3-1 or 4-1 would make for extremely expensive storage on a dedup appliance.
• If you're really considering an appliance, have the VAR/vendor prove their claims --- on your data.
• Consider: if you procure an appliance/solution, is there a method to test the dedup ratio of some of your data?  That is, will you be able to tell the ratio due to your SQL dump of a different/new database object?
  

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