I have two 3TB drives in a RAID 1 on SATA 3gbs. So I did a benchmark for you with small transfers that would fit in the drives cache, as these are going to be the fastest transfers they can achieve, sustained transfer rates are much lower.
If you check the image even the small bursts only get up to 200MBs about 100 MBs short of the sata 3Gbs. You should notice no performance difference with spinning drives between SATA 2 and SATA 3. The volume D pictured above is a RAID 0 with 10k drives on 6Gbs link and they barely push over the 3Gbs mark. I wouldn't worry about performance with any consumer grade spinning disk between between the two SATA revisions. However, certain SSD's can greatly benefit from 6Gbs as single drive volumes can get very close to the SATA 3 limit of 6Gbs.