A decade earlier I was into DIY car performance. I had a Nissan 180sx that was at ~220WHP and slowly tuned/modified it until it was at 380-400WHP. The car could easily kill it's driver if they weren't careful because it was really damn fast. At that point, there were no more easy modifications and gains were in the variable/dubious 10-15WHP that would only be noticeable if I ran the car on a timed trial or a dyno.

How this correlates to my HDD to SSD experience is that my workstation is already fairly optimized ( not overclocked mind you ), so biggest improvement was possibly in random seek and since I don't run MySQL or any other server system on the computer that does thousands of file op's/second the improvement was relatively marginal.