As of now, the BT-821 now utilizes the SiRFStarIII GPS chipset. When the BT-821 was first released, it had a MTK GPS chipset. Depending on when you purchased the BT-821, it will determine whether you have a MTK or SiRF chipset.
You should not see any compatiblity issues with the BT-821 MTK or SiRF because they both output the exact same data and protocol.
On the other hand, there's only a slight difference in performance between the MTK & SiRF, but should not affect your usage.