Various people have some read/write benchmarks of various SD cards on the pi here. Notice that the read speed maxes out no matter what at around 20 MB/s, and the write speed will not be higher. That's a limitation of the pi's SD card reader. It is worth noting because it means there's no point in very high performance cards; you can get the 20 MB/s read even with cheap class 4 cards. Getting a high write is harder, but it is the read that is by far most important to the general performance of the system.
My question is now does a SD card only solution (with the best performing SD card for the RPi) can beat a USB/SD card hybrid solution in speed (boot partition on SD card, root FS on USB drive)?
The hybrid system should be faster in optimal circumstances, since you can get ~30+ MB/s from the USB bus. The only issue with that is your internet connection is bound to be on the same bus, since the ethernet jack is in fact part of it. That maxes out at ~12 MB/s but realistically usually won't be more than 4 MB/s. So the advantage may not be so great, but under optimal circumstances, it could be up to 50% faster.
Of course, the ultimate significance of that depends on what you are doing. It won't make, e.g., browsing the web faster. It should make loading big applications faster, and stuff like browsing images. If you are running some kind of data server (e.g. an NAS), it would be worthwhile -- but in that case, you don't need the root filesystem on USB, just the bulk of whatever it is the server serves up.
