The MOSFET you use in this circuit is rated for 55V at 47A, so why is this product listed for only 5A operation?

For the IRLZ44N MOSFET to switch that much power it would require a lot of heat sinking. When switching 5A the MOSFET stays within an acceptable temperature range. If you could provide good heat sink (perhaps bolt the MOSFET to a piece of aluminum) then you should be able to safely run over 10A.

In your product description you say that you've modified the published circuits (Ken Hewitt's design) to make it fully adjustable to all standard radio systems - what did you change?

We found that the resistor-potentiometer-capacitor combination didn't allow for adjusting the turn-on point over the full stick range. We widened and centered the adjustment range.