is there an xp table somewhere that tells how much xp is needed to level up? something compatible with the uncapper. can those values be changed?

Don't think THOSE values can be changed(yet)..and I haven't seen such a table.. but

http://www.uesp.net/wiki/Skyrim:Leveling

You can use the formula to determine the XP needed for each level.

yes, it's true, thanks
but, if they cannot be changed, how do slower levelling mods work?

By reducing the skill xpgain mult(this example is taken from Elys Uncapper) values. This does create the illusion that it takes MORE experience to level up a skill.

But again, I don't know if there is an existing way to increase or reduce the xp values for skills.

ah, I see. thanks for clarifying that