At those rates, you would probably be better off with a 2 HP 2 speed. Most of the savings comes from the lower speed so a VS will only give you slightly savings but the extra cost could make it many years before it breaks even with the two speed.

You can usually do even better with a two speed motor swap instead of buying an entire pump. Doesn't require plumbing changes either.

Also, with a spa I would not down size because you might be very disappointed in the jet pressure. Most 6-jet spa's require a 2 HP pump if running through a filter and heater.