Regardless of what you do, combat or roleplay, people will only be falling behind those in levels that consistently roleplay with anyone and anything.
For those that somehow haven't seen the XP brhaviour yet, when you are at blind drive, it only takes one level appropriate dungeon per week to remain at a state of switching between slow-down and blind drive. That's genuinely the rate it goes.
If someone isn't seeing that they're still coasting off of other buffers created at character generation or extended periods of time of roleplay over combat, or they're level 5s-10s still killing beetles and skeleton knights and things far below their level they believe are actually for their level for one reason or another.
People that lead a more mechanically challenging roleplay style over a socially challenging one will inevitably encounter blind drive one way or another unless they are a really casual player.
People that spend far too much time in combat will experience a perpetual state of blind drive. People that spend far too much time in roleplay (there is such a thing, unhealthy hour scheduling) will (and have) power levelled onwards and upwards while never seeing what a blind drive is.
The mechanics have been spoken of and even if the numbers themselves are fairly enigmatic by virtue of not being apparent... if you want to level fast, you have to roleplay. There is no getting around that fact.
The combat XP buffer just determines on which side of the central level point you should be sitting at in terms of time spent in game and how much you have roleplayed.
If you want to move the focal point of where you sit for your expected level, only real life time passing, and roleplay XP will shift it upwards.It's just the math of how the system works. Increased and decreased percentages against the combat experience, do not shift that point on the scale, because that point on the scale is only = time + roleplay XP. A percentile buffer for combat XP, negative or positive, is still trapped and locked to that point on the scale.
It's not math numbers that you can see, but it is math theory that you can apply. You have to roleplay if you want to level fast.
Alternatively, make multiple characters and they all benefit from their own XP caps that shift the same amount of time. Technically makes it worth it, but it does split ones attention.
The TLDR graph: