My highest level character is 43 and I never level up QL but still I find the idea of changing it good (from the reviews I read from players).
The best thing would be to make it exponential as when you're at higher levels you need more xp so a greater boost would be better.
Maybe each level you gain a n% xp modificator
n = sqrt([playerlvl] * y)
The y should be adjusted to the number that fits the best.
This way higher level players will gain a better xp modificator when they invest their SP and lower level players will get a 5% modificator (if y equal or higher than 5 (and rounded up to the superior)).
Everybody's happy

?
ps: Sarumar, with this equation (where y = 5) when you'll reach lvl 200 it will give you a 32% xp increase which might be helpful to gain the million (I suppose) xp you need to lvl up.
This will negate the need of investing 20 (which means 80 lvls...) + points in QL to gain a little 200% xp boost.
Maybe it is too overpowered but still, I never played at higher levels than 43 so I don't know.