I have several of the AR300-M and just got an AR750 recently. 10-12Mbps is where they both top out. I’ve heard anecodtals of slightly higher but not much. You can’t expect much for a device using just a watt or two and that can run off a battery pack. I’m satisfied (enough) with the performance and it’s night and day better than any other similar-vein device I’ve used in the past (many of them, trust me.) 10Mbps is pretty good, certainly enough to get things done. If you require higher OpenVPN thoroughput you are going to need to move to a full-size device and even those generally are NOT very good for OpenVPN either. My ASUS models (full size routers) often can only do 10-15Mbps as well! There aren’t a whole lot of them which are capable of any faster via OpenVPN. It’s pretty hardware intensive and not hardware accelerated like other VPN protocols. I’m sure there will be a newer SoC in time that can provide faster speeds in the same power/thermal envelope as the current CPUs these little devices are using, but we’re just not there yet.

EDIT: Forgot you were coming from MT300 yes these do have faster CPU but we’re only talking 650 versus 580 on the same architecture. It’s not going to be any substantive difference.