The thin flat belts work very well on higher speed drives. The yellow one supplied for my Clarkson grinder running at 2,800 rpm on 3 inch (ish) diameter pulleys is more than satisfactory. However a green one bought from a transmission / bearing place for my SouthBend lathe many years ago didn't work very well. Torque transmission appeared to be limited and it was very prone to slip on all but the highest speed where it proved just barely adequate.
Digging into the technical data eventually revealed that torque transmission of these thin belts is very dependant on surface speeds. It seems that stye of belt is generally optimised for high speed duties so technical data should be checked before using in other than like for like or factory / spares supplier approved duties. As I recall it they do vary and there is a reason for the different colours.
I eventually got a leather faced fibre glass cloth backed Stephens Miraclo belt for the SouthBend which was wonderful.
Clive.