You made a big jump on the prop, pitch and diam. On elect. planes, you really have to stay close to the the stock prop,try one pitch incr. up or down, or the same with diam.One step up on the pitch,will give you a higher top speed,BUT with elect. we always have to watch our amp. draw. You can very quickely over draw the ESC,and beta. pack.Going down on the pitch will give you more pull from 0 MPH up to speed, but your theoretical,top speed will be lower.Different size cells of the same Mah. only weigh,more (or less.)On my back yard flyers,I epoxy a piece of breaded dacron fishing line to the lower gear strut just above the wheel on each side and tie a small rubber band between them,give it a little, preload.Yes, low wing planes don't like to slow down, as well as high wing planes.Try to come in quick, get to two feet above the runway, and chop the throttle.

Hope this helps a little.