When ADSL+ was introduced, the minimum guaranteed network capacity, provided by BT, amounted to 100kbps for the average business user averaged over the peak period. Whilst this figure may have increased, it probably has not reached 850kbps (although I was forgetting that VoIP is symmetric and therefore thinking about the downstream limits).
ISPs rely on some users under using during peak periods, or only using off peak, and others overusing, and may have price breaks to encourage under users. Because you are only saturating 850k, rather than 10,000k, you won’t be at the very heaviest user end of the spectrum.
However, you also want guaranteed bandwidth, whereas IP networks are based on the idea that, over the long term you may get predictable bandwidth, but in the short term, you may have to compete with a lot of other users making high demands, so you need to be discussing what proportion of the time the ISP believes they can realistically meet your bandwidth demands.
Actually, even if you were to generate 850k for 8 hours a day, you would be in the range were many ISPs would impose fair usage limits.