Hi.
For the MOSFET's and how close they can be on the pipe, I really don't think you will see much of a heat difference with them close together or far apart. The pipe being copper, should share heat from one end to the other at a really fast rate, and the whole thing should transfer heat to the water at light speed. So don't worry too much about the most efficient spaces between them. As for noise, as long as you keep all the gate wires short and don't twist them all together with other wires, you shouldn't be able to get them close enough to see any noise issues either. The issue with wire proximity is really about touching wires, or possibly long runs of two or more wires that are consistently somewhat close to each other.
Edit: Don't be afraid to get multi-dimensional with your wire layouts if you can. Having say, all the gate wires pulled up, then all the drains pushed down, can give you just as much separation as if you did it left to right or forward and backward. But may or may not be easier to do.