When I say very little reward, I mean the risk is greater than the reward. You can potentially brick your A300 attempting to put an X300 bios on it. Even if you do manage, there's a chance the 5x00G APU still won't work.
It's why you're asking, right? You, understandably, don't want to risk your hardware [and wallet] and that's why your asking here, because the risk is too high versus the reward. I get it, I'm in the same boat, I'm stuck with an A300 with a 2400G. I'd love to put in a 5700G or better yet a 5600X. It's why I'm subscribed to this thread as well.