But anything involving the CPE directly (as in, the traffic stream terminates on the CPE, be it the CPE itself downloading something or acting as Mikrotik bandwidth test client) will not really test its routing/bridging performance as you want to test pushing packets vs terminating packets. Bandwidth test streams don’t cause disk IO but are still slower as you need to ACK on TCP, and do whatever it does on a UDP stream.
To reliably test you’ll have to put something behind the CPE. If that’s not possible, I guess install a centralized router as a bandwidth test server and use the built in client on the CPE, but be aware that the results may vary from what you’ll see when the customer passes traffic.