This was tested using a party with no weapon on the main character, and two fire attack increasing summons. Only Feena was in the party. She was max level, with a single EMP point into her EMP support skill. The opponent was an Earth Test Machine Alpha.
An attempt was made to get in 10 attacks against the machine per crushed stack level. Due to the randomness of triple attacks, 20 data points at 0 stacks were collected, 11 data points at 1 and 3 stacks were collected, but only 7 data points at 2 stacks were obtained.
The averages for damage dealt came out to 1950, 2053, 2128, and 2224 at their respective stack levels. This is AROUND a 5% increase per stack, although it could be 4% or 4.5%. More testing (maybe with a really good grid for added granularity?) could help clear that up.
Not enough EMP exists on the character to test a second star at this time.