Gosh, think it would be really hard to have an overall grading system, as it would depend on two things I guess, the way the bat looks and feels firstly, i.e. grains, pick up, balance, and secondly the way the bat plays (I think people have sometimes referred to this as the ping!). The problem of course if that what would be a G1 bat in terms of look, feel and playability for me would be very different to someone else due to many factors.
Therefore, I think to be consistent you can only grade on looks as without going into massive amounts of technical details everything else would be quite subjective. Just think it is up to the manufacturer to be clear why he is grading in a certain way, and up to the consumer (us) to question it!