So, I was looking into this very same heating approach and one thing I learned (by using RadiantWorks) that hasn't been mentioned so far in this thread is that
to squeeze every last BTU out of your low-temp water, you will need to increase the flow rate (gal/min) of the water through the slab.
If you increase the flow rate through a given PEX diameter, you will increase the velocity (ft/sec) of the water through the PEX. When you increase the velocity, the fluid resistance to that flow will increase by the square of velocity increase. What this means to you is that you will need to increase your pumping power by the square of velocity increase. That means more $'s of operating cost.
The solution to this is to increase the diameter of the PEX so that it becomes possible to increase flow rate without increasing fluid velocity.
So for instance, you might need to double your pumping power, but not quadruple it.
There's another consideration regarding flow rate and velocity...
When fluid flows through pipes, it may flow gently and evenly (laminar flow) or it may flow in a haphazard, tumbling manner (turbulent flow). Turbulent flow is much better for transferring heat than laminar flow.
There is an engineering term to determine the proper flow velocity, called "
Reynolds Number".
There are charts and formulas for determining this.
What I'm suggesting is that designs you have seen that use larger PEX diameters than is conventionally used, may not be so crazy after all, if their reasoning is sound. If their reasoning is not sound, then of course, they are Coo Coo.
So, I'm suggesting going with the closer-spacings as has been advised, AND increasing the tubing diameter, so that you can get elevated fluid flow without unduly increasing pumping power requirements.
-AC