I know it's late in the day, but I just wanted to update this thread. As I said in the OP, the install engineer who sorted my 50 meg was very open and knowledgeable, and had assured me I was in a very low contention/usage area so should see great speeds. I was scratching my head a little and decided to try a different FTP client.
Voilą. Full 203KB/sec upload, rock solid for 16GB of upload continuously. Not a glitch since, so it seems the recent update to FireFTP knocked something out as it used to work fine pre-update. All's well that ends well. I just wanted to add this since others searching the forum might have taken the thread to be another "VM suck, look how bad this is" post. Definitely not VM's fault, working perfectly now.
BTW, are my T3 timeouts anything to worry about? I seem to be getting a lot lately and power levels are creeping up slightly across all four channels.