You answered the cost question - more data consumption and higher service levels are priced into the business class price, which (without bundling discounts on residential) is about $10-$20 more for the lower tiers.
You can push as much data as you want. Comcast can decide you have pushed too much and ask you to terminate (with ETF) or go to a higher speed tier.
I would expect something on the order of "more than 50%" of your total capacity would trigger that. Maybe less, or more. Who knows, it's not a hard limit.--
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