I am using transaction-isolation = READ-UNCOMMITTED to improve performance on a heavy load read write database but I was wondering if there are any potential issues with using pt-online-schema-change in this mode.
There are no ADDITIONAL risks on pt-osc if you are using read-uncommitted isolation level.
If your server have a heavy write/read load, try to avoid to use --alter-foreign-keys-method=drop_swap because the table that you are updating will not exist for a short period of time (between dropping the original table and renaming the temporary table).
Please read carefully the documentation before run it. ( http://www.percona.com/doc/percona-toolkit/2.1/pt-online-sch ema-change.html)