It's a power of squares scenario.
You basically square the wind speeds in question and then compare them (divide).
Detailed formulas can be found for wind turbines or sailing vessels in racing books.

When you double wind speed, you quadruple the power (10 to 20 is 100 compared to 400 when squared).
30 knots has 2.25 times the power in the kite that 20 knots puts in the kite.

Going from 30 to 35 increases the power by about 35%.

The adding 5 knots and doubling force only works in low wind speeds. Actually, from 5 to 10 it would quadruple force.