We're happy you're planning on using our products. I will try to explain everything about your expectations.
As for cost and performance ratio, TRB140 is a great device for your use-case, as it will not only be great additional hardware to MikroTik device, but also will be able to give all the services that we provide on our other models, including remote management of the other network devices via RMS.
Regarding your concern about VPN throughput, it depends on the data itself. If it's not streamable data like IPTV or Video encoding, I believe you should be alright with the current limits of TRB140, and of course, the CPU will give a big performance boost on encryption/decryption.