I have upgraded to 350mbps earlier this week, but not once has a speedtest resulted with a result over 200mbps. I usually run in model mode, but I have stuck it into router mode and tested the speed over Ethernet with similar results.
If you think that you have a BB speed problem, then to check the speeds coming in to your Hub properly, and whether they match what you should be getting, you cannot rely on wifi tests, you need to do it this way first. Make sure your device has a gigabit network card and has the up to date drivers. Use a new Cat5e/6a ethernet cable to connect it to the Hub - which you have put into modem mode (to ensure only your device is connected). Then, boot up your device into "safe mode with networking" (to disable any software on it that could be limiting speeds) and run the test at speedtest.net on 2 different browsers one of which is a fresh download of one you haven’t previously used (e.g. Firefox, Opera, etc).
-------------------- Services: HD TV on VIP (+ Sky Sports & Movies & BT sport), x3 V6 boxes (1 wired 2 WiFi,) SH2 in modem mode with Airport Extreme Router. On VIVID200, Talk Anytime Phone, x2 Mobile SIM only iPhones.
Have you checked under the Configuration tab that you're actually set up for 350? You should have the figures highlighted in yellow below. If you don't, you'll need to phone VM to get the hub configured correctly (although the 20mbps upstream suggests that you should be!). Also, you need to test using an ethernet cable with a gigabit capable ethernet adapter - most newer PCs have these but you can get a USB adapter if yours has a 10/100 adapter fitted - if you test using wifi results will be variable, particularly on the 2.4GHz band.