A couple of times I've removed ASM disks from a diskgroup and found that the rebalance operation will take too long.
This is not a problem when you don't want to disturb online users, the rebalance process only locks for writes 1mb of data at a time, so letting the rebalance to run slowly has no performance impact.
My problem was that I needed to free the disks as fast as possible in order to finish some IO tests within a limited timeframe.
The procedure to change the rebalance power of an ongoing operation is very simple:
alter diskgroup DATADG rebalance power 6;
After that the rebalance operation will restart with the new set rebalance power.
For a complete example check this document: