On my mag sorc Im sat at around 15k spell pen, is that too much? I know diminishing returns is a thing in Eso, just want to make sure I'm not over penetrating (gigidy)
15K is a nice solid number in PvP. Before the changes to shields it might have been to much but now even MagSorcs have to build for some resistance and 12-15K is about what you hit with 5 light 2 heavy. So there will be a few players where you have more penetration than needed but the great majority will be over 15K, making it the stat that gives you the most bang for your buck.
Long answer, as long as players have 20K+ resistances, often 30K plus on tankier builds, more spell pen is going to be useful. However, it is a careful balance between that and actual damage. You could have 30K spell pen but only doing 1K damage with a skill. So while you are negating another players 30K resistance, you are only doing 1K damage and aren't being really effective.
In PVE selfbuffed, yes that is probably too much. In PVP, its hard to have too much. It is always going to be opponent specific, but most people will have more than 15K spell resist. As a general rule, penetration is the best way to boost damage until you get very close to the resistance of your target in question.