Different powerline adaptors support different speeds. I think they can go up to 1Gigabit now but that depends on things like wiring, the quality of the devices themselves etc.
You need to look at it in context too. You have 350Mbp downstream. That's shared between all your devices at any one time in theory. Does the device(s) using them really need the speed?
The question is, what are you using it for? You might find even though it's 40mbits at worse, it's good enough for what you need. The more important thing as well is to make sure it's stable. Better to have 40mbit stable connection than 300mbit that randomly cuts out!