Hmmm.. With something like stats, having permanent modifiers be random I am not sure I like. Even less so about the loss. To just use your example as an example: Losing reactions could be quite detrimental to a tinker that likes fighting, and not idling. How about... Every 8 S-levels, a tinker get either a str or a dex increase. Keeps a randomness to it, but assures that at level 40s, all tinkers have gained as many stat points overall. Losing stat points I quite vehemently disagree with \:\)

Getting slightly sidetracked.. What about every X levels, a tinker gets tinker points, which can be used for whatever. Either for dex/str, at the creation cost, or some other tiny abilities of varying cost. Preferably not very powerful abilities, but some small yet still neat things.