MTK is quite good, but it's becoming worse in the perf/$ ratio, i.e. the SC9863A is a octa core A55 chip at 1.5GHz, while similar MTK devices are dual core A7 at 1.2 GHz. The architecture improvements alone are excellent, not mentioning the extra cores and higher clock speed.

The key is most certainly not the same, because I doubt they would go through the trouble of doing actual secure boot verification, and storing the data in the TEE, and just have the same key. Additionally, the U-boot code I obtained lies to the user about commands not being found, if the command doesn't contain a valid unlock key.