Effects of turning the transaction safety OFF
The main disadvantage of high-performance mode is going to be your possibilities of failover.  Because the communication between the principal and mirror server is now asynchronous with high-performance mode, the only way to failover is with a forced failover and possible data loss.

If you had to do a forced failover to the mirror server, any transactions that weren't sent to the mirror server would virtually be lost.

The real use of using high-performance mode would be that the following two conditions are true for you:


You want to incur the least amount of database mirroring overhead on the principal
You don't care if there is data loss


You should read up on the following BOL reference on Asynchronous Database Mirroring.  It will answer all of your questions more in-depth.