I wouldn't have accepted that on a 350 either though!
You aren't the first and won't be the last to have issues with slow speed on a connection and either think or be told that 'well if you just upgrade to a higher tier then it will magically fix the issue'. This never, ever happens, if there is a fault preventing you from getting the speed you want then upgrading will do absolutely nothing other than make your bank balance slightly lighter and VM's slightly higher each month!
So, first thing. call VM and cancel the upgrade, assuming that you are in the 14 day cooling off period, they have to action this. Secondly, let's see the condition of the your connection, can you log into the VM Hub and look for the downstream and upstream statistics and the connection logs? Instructions for doing this are somewhere in a number of posts on this forum. Copy and paste the results into a new post on this thread and we can take a look at what might be going on.