The xp for a goal is just a multiplier of the xp for an encounter at their level right?
In which case, the number of "medium" encounters to get through a level is roughly constant until level 10. (xp for a medium 1 person encounter divided by the amount of xp to level up is approx 15 for each level 5 through 9). Level 10 takes a bit longer at 17.5 encounters, then it drops to between 9 and 10 for levels 11-14.*
So if it is going to be an issue, it will probably show up when people hit lvl 11 and others are down at 9 or 10. Assuming players that complete goals at something approaching the same rate anyway
*encounter numbers from kfc, level totals from the roll20 compendium