They don't really offer a 350 service, they always give a little over in their modem configurations to allow for overheads and channel bonding and whatnot. By the time you get to 300 mbit, that little extra adds up to another 50 mbit. I guess it helps people get the headline speeds on the test sites so customers don't complain they aren't getting what they pay for (congested areas and Puma 6 issues excepted).
As others already mentioned you're wrong mate. They advertise the service as VIVID 300 but the service goes live in your account and router as VIVID 350.
Upgraded my mate last month for him, even the live chat agent insisted they only did 300Mb, I got her to go ahead with the order anyway...
What do you know, service went live and his account shows VIVID 350, then with the new hub and profile, he achieves around 380Mb!