I discovered ~450uA additional current draw coming from the STM32F101 in Stop mode. This condition occurs when an external pin interrupt from a peripheral goes active during Stop mode even though the NVIC is disabled for that pin. The key is that BOTH the NVIC and EXTI must be disabled. As soon as I disabled the EXTI for that pin as well, the power consumption was much lower as expected. I don't see this occur on my STM32F103 eval board so it must be something inherent to the STM32F101.