I don't think you can apply the same metrics to a PC as to a mainframe. The former is used primarily for single-user interactive work and so it is instantaneous performance that is important. That would seem to indicate that an upgrade to SATA 3 would be worth it. However, tests have been done and show that this is not necessarily true. For example, see: http://www.tomshardware.co.uk/ssd-upgrade-sata-3gbps...
Prsonally, I wouldn't expect any significant increase in performance just (very) marginal reductions in program load times. As to the particular combination of hardware, you'd have to try it and see. On the other hand, £20 isn't a fortune.
Don't blame me - I voted Remain