The conversion from RGB scart to yuv/ypbpr is a fairly simple addition/subtraction, and not much more.
I dont think it does anything with the sync signal, as the ypbpr will accept the 15Khz, but it may modify it slightly, to make it more compatible with more TVs
It doesn't stretch the image, the tv does, same as if you were to connect scart, or the GBS, directly to the TV.
I'll have more of a fiddle with it over the weekend.