I raced for a number of seasons a club 500, and we raced to the original idea, stock. As mentioned, these are not fast hulls. This might sound off track, but some of the issues you are facing is due to the rudder blade that's comes as stock being too wide, that's what your pal is suffering with, one way is better than the other due to torque roll. If you experiment, keep the length the same, but slowly make the blade narrower, this will improve turning.
They do porpoise, and moving the battery tray slight movements forward, from the most rear position will help. I recall, on choppy days I moved it forward up to an inch, on calm days had it back. People running heavy boats didn't suffer this, as they sat lower in the water, and obv went slower!
My boat was dead light, no excess weight, and won the season champs a few years running, however, it did suffer with porpoising at speed, which you can cure as above.
If you look at this video (crappy quality!) the boats are hopping around, my boat is later in the video, the gulf blue and orange colours, number 40, it was the fastest boat, and you can see its hardly porpoising, just due to battery placement moving the cog
https://www.youtube.com/watch?v=dXYyD3xjGn4&feature=youtu.beI had forgotten how slow they where! stock Johnson 500 motor, and nimhs batts!