Firstly, the tooltips for the two are wrong - they still indicate just a +5% bonus. Given the now variable nature of the stimulus/festival, either correct this with the formulae used, or simply eliminate it.
Secondly, the values seem *VERY* step-function. I'd have to look at the code, but it *roughly* appears that the formula is something like "SOCIAL production / 10" dropping all fractions, times 5%. AKA: (SocProd DIV 10) * 0.05
Social production:
0-9: NO benefit from the festival/stimulus
10-19: +5%
20-29: +10%
30-39: +15%
That's terrible. It should be a much smoother gradient. (Social Production DIV 2) * 0.01 as a bonus , is a far, far, far better idea.