I would have to agree that for MOST users this apparent change from 3Gb to 1.5Gb SATA is a non-issue and should make little difference in real-world operations. However...
I know that when I upgraded my 1st generation Mac Pro to a RAID 0, two drive set that the only differences I saw in real-world performance were in large file copies and duplication, in some Photoshop operations on large files (history and caching), and in exporting large video files from iMovie. In some cases the differences in time were very substantial but they seldom equaled the raw improvements I saw in simple disk i/o benchmarks.
However, I saw no differences in boot times, program open times, or in the vast majority of operations in Photoshop. I also attempted to see if virtual memory performance had been affected and I was never able to find a test that revealed any improvements.
The reason why I mention these tests is that they are real-world results in a configuration that would bear some similarities to the difference you would likely see between running a FAST SSD over 1.5Gb versus 3Gb SATA. However, RAID HD sets and SSDs have different characteristics and the biggest improvements I saw were in write times which are an area where SSDs do not show as great of gains (SSDs are stronger in reads and random access times, with the latter being by far the most significant in terms of improvements over HDs).