Have you tried various speed sites?Just upgraded to Ultimate Oomph with its M500 broadband and although a couple of speed tests on different sites have hit 550/35 its mostly 220 - 350 Mbps as per the speed test below, presumably due to over contention:
This is in wired connection in modem and non modem modes and all power/noise levels are good.
I upgraded mostly for the extra upload speed, SIM (to use in 4G backup modem) and Sky F1 HD. However it appears the contention issue that used to badly limit our 200Mbps package a couple of years ago until it was fixed is still there, it just takes a faster connection to show the the new level.
If this continues I will be complaining...
Also what about downloading from a known fast server to check your download speeds?