The skills mods unlocks the ability to purchase/upgrade any skill without unlocking it by putting points into the tree. It also unlocks all the slots. (You still have to earn skill points, it just gives you more options to spend them on from level 1).
With the XP mod:
If you are level 5 with the mod, you would be level 6 without.
If you are level 10 with the mod, you would be level 12 without
If you are level 15 with the mod, you would be level 18 without
If you are level 20 with the mod, you would be level 24 without
If you are level 30 with the mod, you would be level 35 without.
I think that's a little TOO slow progression. In BC it is level 30 is level 33 and that's a good balance. With level 30 to 35 you will reach some end game armors only if you absolutely do everything in the game.