So to be clear here, your not that bothered about hitting the heights of super-fast broadband?-its more stability in terms of latency that allows your radio streaming to work and web pages to work?
When the engineers have called, did they demonstrate to you that it worked on
their machine-did they get the speeds, web pages, radio streaming ok?
As for your logs, they show that from the modem point of view,everything is fine.The lack of anything in the T1,2,3 and 4 timers would suggest the connection from the network is pretty solid and the downstream MER's are good as well.
Would it be an idea to use something like Ping Plotter to ping the UBR ( your WAN default gateway) over a day or so and see if you can note any dropouts? It seems to me the speeds are there or thereabouts but it's the stability that is causing you grief
PS-should have mentioned it's quite good for latency as well.
HTH