Personally, I do think xp gain on Kymlun is on the high side, and the change of focus from combat xp to roleplay xp is a good move. Monsters and dungeons can be rewarding enough through items they drop, or the way they increase a faction's opinion of PCs.
Level 40 could take a year to reach. Level 20 can take a year all of its own, as far as I am concerned, though how long a year is depends on how many hours you can put in each day, and how you spend them.
There's also a tentative experiment with roleplay xp underway already, and tiers are a good way to allow DMs to influence their rate (but carry a risk of favouritism or simple mischance due to timezones).
But when it comes to balancing xp gain, there are a lot of options with no clear way to get it 'right.' In broad categories, the individual's effort, others' judgement and an impartial time interval are how experience can be gained. We can play with those, and clearly, what you reward most affects which activities players get involved in. So far so good.
All of these categories can be high or low, and can be further tempered by xp drains. This affects how soon a character gets from level 1 to 40, or in other words, how long they'll be influenced by those xp rewards. We want characters to progress at an enjoyable rate, which means neither making it impossible to reach level 40 nor letting them reach level 40.
Problem.
Luckily we have non-xp rewards like character reputation and equipment to stretch their progression beyond that level.
Unfortunately, the further a character progresses in power, the bigger the gap between the oldest players and the newest players becomes. If the gap is too big, new players feel insignificant, can't play their characters from level 1, get demoralized. If the advancement rate is high, players can quickly catch up even if they don't have hours on every day of every month to grind. This means however that xp is worth nothing at all, and all your progress is no achievement to be proud of.
A middle road is hard to find, as player opinions differ on what is or is not speedy. The one solution I know of is to retire old characters at some point, either through old age, permadeath, ascension or some other means, but that involves forcing regular players to give up the PCs they enjoy and worked hard on. It may be good for them to start something new, but I don't think we could get away with it
Left alone, going from high xp gain to low xp gain is actually going to make the gap bigger and we'll be left with the 'powerful' PCs from the days of yore and the hopelessly weaker new generations.
Long story short, radical changes to what activities are rewarding should be no great problem, but radical changes to the advancement rates needs a lot of careful consideration.