Hey Steve, thank you. Regarding the missing highscore saver, it´s the same thing like with Space Firehawk:
Sub Chase 64 is intended to be a arcade/cartridge game, so I haven´t implemented any disk operations.
But if you really need a high score saver: The guys from Laxity have built it into their cracked version, which can be found on csdb.dk
Just turn off the trainer ;-)