Is it possible to apply an XP multiplyer? I've beaten the game on Hard, and I want to do a sort of NG+ playthrough, so I would want to make the multiplier ridiculously high so after a few quests I could reach the level cap and feel like my old witcher self again.

Haven't tested it, but I believe you can do it by editing abilities/geralt_basic.xml should do it. Change the "1" to whatever number of bonus points you want to get each time you level up.
# Various group
<experience_bonus mult="false" always_random="false" min="1" max="1" type="bonus"/>

I've tried that and I saw no noticable effect at all

Anyone else?

No?

What did you set it to?
I tried min="100" max="100"
At the very beginning of chapter 2, and I leveled up 6-7 times just going through the fog killing draugirs. Every time I killed a draugir, I leveled up.

I set it to min/max 1000000 right before I went to finish a quest, but the reward was still the same. Perhaps that only affects killing monsters?

I'll try and set it to 100000000 and clear out a nekker cave

Apparently it does only affect killing mini-bosses and bosses (haven't gotten to a boss to confirm). But so far, set at 100, killing one draugir = level up, killing the ghost of the Dun Banner = level up, attaining knowledge abut monsters through combat = level up.

It may be that the number you are setting to is too high. Try 999 first.