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.