Yes, SSD's are better served from a sata3 port, 6Gb/s.
Yes, a single 7200rpm sata3 (6gb/s) drive will perform just as well on a sata 2 port as it will on a sata 3 port, so yes, a sata2 drive will perform almost identical as a sata3 drive in real world performance. You will only see a difference in synthetic benchmarks.
Sata III HDD on a SATA III port will only improve burst rate. This translates to very little real improvement in day to day performance.
HDD performance is limited by RPMs and platter density and even at 15K RPMs it is not going to saturate SATA II.
More of a marketing tool, as manuf state support for the newest/fastes interface - Fine print left off, doesn't improve performance.
While SATA III SSDs benifit from the SATA III interface - it's primarily in the Sequencial read/write performance with much smaller improvement in the more important 4 K random read/writes.