EXACTLY THIS. Timed XP potions encourage nothing but zerg-rush XP-min playstyle. It drives off players and discourages long quests. Think of all the work that went in to ToEE vs. how much it actually gets played.
That could be a good approach, but maybe not set numbers? The percentage-based grants DO encourage tackling hire difficulty to maximize that first-time bonus. The devs have stated they want players to try higher difficulties and have attempted at various times to create mechanics that encourage this.
The XP potions as-is kind of work for and against it. On the one hand, yes, I'm going to chug a potion right before I finish that first-time, high-diff quest... but then i maybe dont want to do that next quest in the chain because its long. Or I'm going to bail on a run that isnt going well "because I have a potion burning".
I'd like to see it be percentage based on total XP earned. So like "for the next 100,000 XP you earn, you get 50% bonus XP". Kind of like the bonus slayers, but not timed. So for each point you really get 1.5 points. No time limitation, its just based on how much XP you earn total.
This would still let the XP/min zerg-rush crowd do their thing, but let the flower sniffers also get full benefit.