I was with VM for 2 years, I had 100Mbit and it would only congest at weekends, between 40-70Mbps... but I was happy with that because where I lived the fastest FTTC speed I could get was 20Mbps - so before you leave you may want to check how far you FTTC cab is away from your home, also factor in things like what the quality of the phone line is like, if FTTC take up is high in your area (resulting in crosstalk - thus even lower speeds again)
Im in my new house now and VM isnt available so Im on PlusNet 40/2 - I get 37.5Mbs / 2Mbps / 15ms (best manually pinging to BBC in CMD Prompt than relying on any testers as they are usually always out)
, mine is full speed, and the MAX I can get is a sync of 69MB which would yield about 65Mbps (and 18Mbps up if I went for the 80/20 package), I live about 200 meters away from the cab, a lot of people experience sync speeds dropping over-time, this is usually caused by more customers taking up FTTC in the same street(s), this can cause crosstalk ingress due to the higher frequency's used in VDSL, this causes the sync speed to dip.
So as you can see.. you could jump ship to another ISP and end up with even lower speeds than your "congested 150Mbps", BTwholesale ISPs also share bandwidth, so your not only on a shared pool of bandwidth through your ISP, but your also on a shared pool of bandwidth with MANY ISPs - yes if you go with a reputable ISP chances are you might see solid speeds 24/7 - but not all are like that.
As its been mentioned VM are working on a fix for the SH3 latency issue due to the Intel chipset
maybe find out exactly what your speed estimate would be on VDSL/FTTC before you do anything.