Did you notice that the law of diminishing marginal returns is applied twice?
First, as the level increases, the amount of science that has to be poured into research is higher.
Second, the effect of a breakthrough is less important.
For example, if you have weapons lvl 1 and you discover weapons lvl 2, it takes 144 science and makes a 100% increase in weapons efficiency.
If you have weapons lvl 2, discovering weapons lvl 3 would take 288 science and makes a 50% increase.
And so on for higher levels.
So in effect we have the (law of diminishing marginal returns) squared, or ldmr²
So I suggest the increase of efficiency should be between 20%-25% with each upgrade.