I'd made some test, which show me that ASM volume files (ACFS) has only one I/O thread per volume file. This mean that, if you have 1Tb volume file and you want to rebalance it you can do it with only one I/O thread.
I execute this (alter diskgroup data rebalance power 10) which contains only database files and has 24 disks. Through iostat I can see that, all disks was loaded by many I/O operations!
But if I execute (alter diskgroup acfs rebalance power 10) which contains only 2 volume files, 1Tb each and has 24 disks. Through iostat I can see that, in one time loaded only one disk! And after some time later another disk was loaded, but always in one time loaded only ONE disk!
It is shows me that asm rebalance for asm vol file do by single I/O operation compared rebalance operation for rdbms files which going with multiply I/O operations.