In response to a question from Joe Elliott:
You inquire about insulating the suction line from the evaporator to the
firewall; I will assume that you mean from the evaporator through the
firewall to the suction (low pressure) port of the compressor. Your logic
is almost correct but subtly flawed. As you say, insulating the suction
line should ultimately reduce the temperature of refrigerant entering the
condenser, but that's a good thing. Here's why. It does lower the
temperature gradient between refrigerant and ambient, and thus (using
"Ohm's law") reduce the heat flow out of the condenser (bad). But it also
lowers the amount of heat that must be dissipated--the absolute temperature
of the refrigerant times its specific heat (good). The good outweighs the
bad, as can be seen from the limiting case: imagine refrigerant already
cooled to ambient--it will provide no gradient for the condenser, but the
output will be at ambient--as cold as you can hope for. As the incoming
refrigerant temperature increases, the thermal efficiency of the condenser
increases, and so does the net temperature drop (as you speculate). But
the increased temperature drop is less than the increased incoming
temperature. Thus the output temperature rises.