I'm curious? Why are all you guys using pumps?

Second question. How big of a plane do you think 2 Saito .40s should be able to pull around? I was trying one out on my Hobbico Superstar trainer to see if it would fly a .40 size plane around. It flew but not well.

I tried a few different props.
11x4 it was turning 10.4k
11x6 was around 9200

I was hoping they would have enough ummph to fly a twin stick.

TIA
Micheal