BP is collecting 15,000 barrels per day and by next week would have the capacity to collect 28,000 barrels per day.
28,000 barrels per day is higher than more recent estimates of the size of the leak. It appears that the current cap method can be scaled up so that eventually the valves can be closed and 90% of the oil can be collected. When the valves are closed then we will know how much oil was leaking based on actually collecting the 90+% of the leakage.
The amount that is collected at that point next week (or in the following weeks if the amount is over 30,000 barrels per day) would be based on the higher flow rate after the pipe was cut.