Actually there is another important parameter that is faster on SD than a HDD, it is called "seek time", it means the time to a information be found and read from your device.
Considering that the boot phase of a operational system reads a lot(really lot) of small files that could be spread on HDD area, SD gets it main advantage because it is based on a random-access memory(it means that you can access any region of the memmory with the same time, not as a HDD where a phisical sharp nedle must seek the disk surface for the information, spending a lot of seek time).
It would improve your OS speed in many ways(boot time for example), but keep in mind that it is still not possible to do this on Windows(it will only be available on Windows 8) you may only test and use it on Linux distributions.
Also, if you want to test reading speed or the seek time of a device(HDD or SD) I wuld recommend you to use HDTune. And if you want to use a SD card to use your OS, be aware of it's class(the higher it is, faster will be read and seek time on it)