I think I now have proof that the file transfer rate is pretty irrelevant for the OS performance on a SSD and that all the performance boost comes from the fast access time.
Here is the setup that gave me the clue:
1. I installed Windows 8 in VMware running on my desktop M4 SSD
2. Then I copied the VMware folder to a USB attached Mushkin 60GB SSD
3. I ran Windows 8 off the Mushkin attached via USB3
4. The I ran Windows 8 on the Mushkin via USB2
These are my findings:
a) there seems to be absolutely no performance difference whether I ran on USB3 or USB2 or from the internal M4 SSD
b) what was really surprising was that the boot time (as to Event100 in Event Viewer) was always around 35 sec from the internal M4, but it is 25 sec from the USB3 AND USB2 ports
c) there are a few adjustments that the system and VMware seem to make when you move the VMware folder around. The system made adjustments the first time it detected the moved folder. That was automatic and was only the first time.
VMware seems to make adjustments when you do the first shutdown from the external disk. The Windows 8 shutdown itself was 3 sec (Event 200) but VMware took a couple of minutes to shut everything down - both when I first attached to USB3 and then again on USB2. But that is also only on the first shutdown.
Conclusion: Regardsless whether you attach the SSD on a 5Gb, 6Gb or 480Mb channel, the performance is the same. The transfer speed does not seem to matter.
Installing the OS with VMware on an external SSD is a nice and very flexible option.