With Billy's concept, you would be able to achieve 99 in the same time frame as you do now.
Your XP would increase as you level.
The Next Statements are only Examples
This would mean, you will not gain 5 strength levels from killing that first chicken.
But, you would also not need to kill 15k Undeads to get from 98 to 99 strength.
It would sort of be like an up curve in XP rates as you level.