I made my own 2whl kit on my 97` Summit and it helped on laying the sled over on hardpack, Now I did notice that if a sled runs a 2 whl kit on a skid that has rail extensions it will be more effective if the hyfax`s go all the way back.
I had 136-151 extensions that did not run the hyfax`s back to the rear wheels and when the nose was in the air when climbing it was very unstable and felt like a unicycle so I reinstalled the outer wheels and it got rid of this problem, This year I have changed skids and am running a full length rail and will reinstall the 2whl Kit again but I`m going to run 3 whls in the center for more support.
This probably isn`t something that every one would experience but is what I have ran into.
Wheel size helps on the rolling resistance, and especailly effective if the rear wheels are the same diameter as the drivers or bigger, but to big and you loose track length.
Good Luck.