Would I ever have to pay the Treaty for commercializing products developed only from rice obtained from IRRI before implementation of the SMTA?
No. The conditions you agreed to at the time of the transfer remain in force now. If you obtained rice from IRRI before 1 January 2007, the material is subject to the terms and conditions agreed at the time, which could not have been the SMTA. This means that the conditions in the SMTA that trigger mandatory payments cannot apply. Of course, if your product incorporates genetic material from rice received with the SMTA as well as rice received before implementation of the SMTA, the conditions of the SMTA do apply to your product, including the trigger to mandatory payments.