I believe soft caps are not defined on salary but on (weighted sums of?) skills.
I second that.
I don't think you can say the allstar cap is at 50k (as said in that thread), seeing the huge amount of allstar players at a pretty young age who are above 50k. I didn't experience any slowdown yet except the part that my C gets older (23yo already), he's at 63k at the start of the season and still he trains about even as my 32k C who's 1 year younger.
I don't think it is salary per say either. However, the 50k is there to say that all-star players can hit the soft cap as soon as 50k. Empirically, it looks as though it is the sum of is, id, reb and blocking (with perhaps slighter weight for blocking) that leads to the soft cap - somewhere around 47 or so. For shooting guards, looks to be principally a combination of js, jr and od with the soft cap around 39. I imagine there is a more complicated formula, so that each skill that figures into salary is considered.
I think what sfg meant in terms of efficiency was on how to figure out where the soft cap is and train other useful skills (say rebounding, is for a sg) before adding that last level of od to hit the soft cap.