Decrypting the sprites was easier than expected. I've done Raiden Fighters 2 for now, which is the first game to have completely decrypted graphics. I still have to do Raiden Figthters Jet and the SPI - I don't yet know if the encryption will be similar.
The game looks good now, though there are still some graphics issues - most notably priority and alpha blending. I will probably not look at those when I've finished with the decryption, leaving to someone else to finish the driver.