For example, the PP addition is clearly for non-spell users, the quantity is poor, I suppose that for spell users spell adders is used instead.
The bonus for SCSM is very poor, so it doesn't reward for time and/or cost of the item (+2, +4, +6...), personally I would change it to +5s instead +2s.
I'd change too the category bonus (continue reading) to +5s, so if you create an item that bonus 'throw weapons' it is more usual than create an item that bonus an specific 'throw weapon', but bonus based on +2s is poor. But THIS ONLY for categories that are type '1 skill per...' like weapons, directed spells, etc., in categories that are different skills use the +2s, because those skills can be very different in the same category. So, the conclusion is that you use them like a skill, not a category, because they are really variants of the same skill, but are not really different skills.
But, how about bonus to BAR (many users want it!), increase the casting level (so you can cast spells greater lvl than yourself with no SCSM and with more duration, etc.), store capacity (like magical staff), etc.?. Nowadays we only can imbedding spells that user doesn't have (or can't have) to expand its casting skills.