In the manga Zamasu states that SSJR is what happens when a god transcends Super Saiyan God level. That doesn't mean that the form works like Blue or any of the established SSJ forms. It simply means SSJR Black > SSJG level > SSJ Black.
Regardless, the multiplier has to be pretty low. SSJ Black was already getting the upper hand on SSJB Vegeta, which was complete bullshit in itself, but he at least had to be close to SSJG level for that to make any sense. Then we see SSJB Burst Vegeta stomping SSJR Black pretty easily. So the difference of SSJR and SSJ is a lot smaller than SSJB and SSJG.