This was kicked around Tomshardware many times and since we'd have to change the laptop to change the SATA port, it's up to you to decide. Let me share that SSD removes seek times so my oldest iron is a 2006 Dell E1505 with 2GB RAM, Windows 10 and an 120GB SSD (cheap.)
That laptop boots and get a google web page up from COLD (not sleep) in 33 seconds.
To me this is proof enough SATA 2 would be enough.
i have a new ssd samsung 850 and it will only work to its full potential with a sata 3, how much performance and i losing by not running it with a sata 3 will the upgrade be worthwhile.