I'm at level 70 and have most of the variants that I want, and I find the grind to be horrible even now. Of course we don't want to tank the economy, so let each successive boost be half as effective. e.g.: Use one 12hr boost, and one 4hr boost: gain 3x XP for the first 4 hours, then the standard 2x rate for the remaining 8 hour duration of the 12 hr boost. Use one 12hr boost, and two 4hr boosts: gain 3.5x XP for the first 4 hours, then the standard 2x rate for the remaining 8 hour duration of the 12 hr boost. Another measure to preserve in-game economy: let some percentage of XP boosts give the full rate even if stacks (but for a shorter duration). So whether you get it from a gift, login bonus, or with theonite, give it x% chance to be stackable with no penalty. To sweeten the pot, increase the odds if you purchase with theonite. e.g.: Use one 4hr boost, one special golden boost: 4x XP for 1 hour, 2x XP for 3 hours. Main point is to let us multiple XP boosts simultaneously to reduce grind. Obviously the numbers, rates, durations, multipliers, etc. would be adjusted as necessary based upon data that we don't have access to.

Yes, it will be very effective. Also after the accolades, I have around 10 12-hour and 36 4-hour XP boosters.