The manual states that the clock runs at about 50Khz depending on oscillator, they're probably basing that figure on a 4MHz oscillator. At 8MHz I would expect around 100Khz.

I'm currently using SHIFTOUT on a device running at 64MHz and just measured the clock frequency to around 320kHz. The reason it doesn't scale 100% linearly with oscillator frequency is, I guess, that they're always holding the clock active for 2us (or more if you set it).

If you need more than 300kHz then you need to move to hardware SPI. Clock frequency isn't everything though.