There is another factor here. I was going from a fast, 3.5" desktop drive to an even faster RAID. In the case of a notebook experience you'd be going from a relatively slow 2.5" HD to a very fast SSD so you'd likely see a greater difference in user experience.
However, for the case of the 1.5Gb to 3.0Gb shift on an SSD you'd be starting with an equally fast SSD that in real world terms may or may not be affected that much by the interface. Benchmarks will show a difference, but in the real world I don't think we've seen any results which show what the difference would be. No doubt a FAST SSD on a 3Gb interface would be better -- but how much better?
1. I went from desktop drive as well (not one of the best, but better than in laptops).
2. Let's assume opening an app takes 10 seconds. Using SSD like in Air shortens this time to 4 seconds. Air's SSD achieves reads at ~80 MB/s. Intel's SSD are more like ~160 MB/s. There's no other bottleneck, so using Intel's SSD launching that app will take 2 seconds. This is just a made-up example, but what I'm trying to say is that even with SSD, you still have to wait for something to load sometimes - and this wait is shortened with any speed increase. Someone earlier said 98% of users won't ever use it at full speed. I'd say most users won't use it 98% of time, but when they do - at tasks that would usually made them wait - they'll notice it.