Slightly lower speeds with 80%. Up to 3 or 4fps. Other than that, can't think of drawbacks. I could care less with P&Y spec on it.
You would still be put in the books but with a * by it to indicate you had 80% letoff.
What P&Y should have is two ** by it to indicate compound kill with 80%, with only one * by 60% letoff, and none for a bow kill with trad archery equipment. I think if Pope and Young were still alive and kicking, they would approve of this change.