Unfortunately there aren't too many high performance RAID cards with eSATA port-multipliers - the interface is too limiting. In other words, to get significantly better performance, you'd need to change the controller and the boxes.
Most people end up with MiniSAS for direct-attached storage if the number of drives is 8 or more.
8-bay boxes start at about $600; 16-bay - $2K. A decent controller - $700-900 (Area, ATTO, LSI).
Yes you can re-use your SATA drives; the data on them is unlikely to migrate to the new setup intact - so if you have anything critical on them, back it up.
You've got a total of 18 drives, correct? 10x 1TB, 8x500GB? Would you be open to getting one 24-bay enclosure for all of them, or would you prefer to get two - an 8-bay and a 12- or a 16-bay?
I've had great results with iStoragePro (division of Ci Design) boxes in terms of quality and support - and also was impressed by Stardom SohoTank. Will be happy to do a quote for you once we zero in on a configuration.
The Stardom SOHOTANK (8 bay) ST8-U5 features a 6Gb/s backplane, we're getting sustained read/write performance of 830+ MB/s in RAID 6 with the ATTO R680 6Gb/s card with SATA drives & DV411 would be the place to buy.