I think the OP was about courses and not speeds.
Snowboard races have been held on real downhill courses in the past.
They were scary to watch but I must agree the falls were less scary than ski falls.
Dog Coughlin was a favorite of mine to watch, and Doug Lewis, ex- Olympic ski downhiller, perfected snowboard downhill for a short time in wind tunnel testing and FIS downhill courses.
I have read that they were discontinued due to insurance considerations which is a shame. Snowboard downhill was a great event to watch and yes, many were held on FIS ski downhill courses.