The distance between the DUT and first expander is limited by the maximum round trip flight for a cable that will carry STP traffic is 20 dwords. The distance between the first hop from LSI SAS3008/SAS3108 to expander is limited by 6.3m for 12G and 12.58m for 6G assuming a cable with a propagation delay of 5.3ns/m. But the distance between the first expander and downstream expanders may be greater. There is no issue with SAS HDDs.