Yes indeed there is a "sweet spot" for industrial efficiency spending. Unfortunately, it is different for each country, and different for the same country depending on the economic circumstance. It is also different for each individual resource depending on what your supply looks like, and what you want to do with it (export/cover population use etc.). You will have to do the math and figure in your own regions situation, and your own overall goals. Personally, I just plug Paul Li into my production minister slot and leave him alone.
2010 is like that in many areas. It is one reason the game is very re-playable, rules and tricks for running one country do not always work well running a different region. As with other situations within 2010, the best advice I can give you is to play around with it. Experiment and discover what works best for you. There might be a situation where you need the cash more than the production boost, or vice versa.
P.S. Oh!, You wanted the short answer! Best guesstimate I can give confidently is somewhere between 60%-70%. Mileage may vary!