The heat is lost through the top of the pool. The wall foam will not help with heat retention. It can make the walls softer feeling but......I don't feel my walls that often so it is better to spend my money else where.
Make sure to go ahead and plan of hard plumbing your equipment. Use unions on everything for ease of maintenance. Think..if I have to take the top off of the filter, work on the pump, etc and put the unions there. Valves need to be placed every where the water comes out of the pool.
I also put about 3 inches of rock in the area under the skimmer and return area. I put pavers on top of that. I sloped it such that WHEN the skimmer overflows it will not wash out the pool bottom.
Kim