In fact the drop would be much more drastic than that due to the fixed propagation times in the CAN controller and the CAN transceivers.
At lower speeds the distance drops to one half when the speed is doubled. However the maximum distance for 500 kbit/s is 100 m, for 800 kbit/s 50 m and for 1 Mbit/s only 25 m (without optocouplers, with optocouplers only 9 m).
The problem is that the total propagation time has to be counted twice when processing the ACK bit. Thus, even if the CAN controller is overclocked, the internal propagation delays remain the same. With typical CAN controller delays of 50 to 62 ns, transceiver delays 120 to 250 ns and cable propagation delay 5 ns/m, you could not reach very far even at 2 Mbit/s.
With 50 ns controller and 120 ns transceiver delay it would take at least 340 ms back-to-back propagation delay even if the devices are wired directly to each other and no optocouplers are used. This might be just enough for a 500 ns bit time (2 Mbit/s) but I really don't see how anything more than that could be used.