I would try to keep it at 1.2 GHz (or at least higher than 400 MHz). Some tasks are still single threaded in RouterOS, eg a single TCP connection with queues / firewall will max out at around 600mbps at 1.2 GHz (example http://forum.mikrotik.com/t/ccr1009-7g-1c-1s-single-stream-tcp-performance-limit-with-queues/117121/1) so at 400 MHz I expect it will be lot lower and not able to take advantage of your 500mbps connection.