I don't know of any way to ask for a video mode with a specific refresh rate. If you ask for 800x600x32 and your monitor supports that resolution at both 60Hz and 75Hz, I'm not sure which video mode you'll get. It looks to me like you get 60Hz.
Perhaps in the future a requested refresh rate can be added as part of the VideoMode structure.
Of course, I'm fairly new to SFML myself, so I could be wrong.