The only thing that matters is how snappy the GUI is, try measuring framerates of the change from 2D Gnome to 3D Unity. Also compare open source drivers vs proprietary at rendering the GUI. Users don't care about how many bits a hard drive is transferring per second as they will never notice.
Users do care about data rates to/from a hard drive. Ever install a huge game? Ever try to play a movie from disk while uploading photos to picasa? What about backing up data by copying between hard drives?
I can all but guarantee there will be complaints about how long it takes to copy 20GB of crap between drives. Or the fact that the video is stuttering as thousands of photos are being accessed for upload. You'll probably hear "This computer is really slow" when it's actually the hard drive as a bottleneck. Better throughput and smarter accessing/layout aren't things a typical consumer will talk about, but they certainly will appreciate.