I played on yesterday, keeping the 15 level increase - it's possible to play, but damn hard :-) I really needed to use the weak AI on a lot of occasions to get through, gaining points here and there to level up 3 times ,now level 23=8. I also avoid gaining points with smithing/alchemy and so on, so pure gains from actually playing the game as a one handed fighter with archery/sneaking mixed in (otherwise I didn't see how to beat anyone at all, need ranged combat as you die to quickly in melee).. With this I could actually beat a couple of bandits at Valtheim Towers and finally the Draugr with the Dragon Stone by running around this hall all time shooting like 100+ arrows to take him down :-) I also for first time used a race bonus of my redguard (adrenaline rush), which was nice. I never bothered before as you win easily without using it - now you really need to use all that there is. It's definitely a huge challenge with this setup of +15, bordering on frustrating at times. I guess maybe 10 or 8 levels is a more balanced way to try.