We already have a 30% xp boost when below maximum level, but what about gear; gear for all the classes are what you really need, having to spend a lot of time doing that kind of farming would be enough to get to level 20 at that stage with proper equipment; a 30% increased chance of gear drops (enemies have to be weaker or above power level to avoid the redtail farming from getting crazier) from enemies would help people get to level 20 faster which im assuming is the point of the 30% xp boost; but because at that point gear becomes a limiting factor you still have to do a bunch of grinding.
Most level 20s I see leveling other classes often have shard gear on there low level class. Shoot, I know of a level 5 warrior with a shard 5 weapon. Gear seems to be the easy part.
if you have people to do shards with of course