That is my whole point, the basic difference betwen the 65c02 and the 6502 was to remove all the illegal opcodes (since otherwise there was no speed improvement). I have no idea if the pre-65c02 roms had any illegal opcodes in them, but one can be 100% sure without some testing, at least some game software did used the illegal opcodes in the 6502 and stopped working with the new cpu/firmware.