Poor A/C performance in Hot Weather may be due to the A/C high pressure cut-off switch being set too low. When the high side pressure gets above about 425 psi, it will cut out and not cut back in until 200 psi. You would need to change the switch on the compressor. 4 pin connector, black casing is stock. Scharader valve on it so you don't have to discharge the system to replace it. There is also a pressure relief valve (Dump) that dumps freon if pressure gets too high, usually around 550-650 psi.
Here are the specs:
Black casing switch - YF1Z-19D594-AA - Cooling fan ON - 310 to 340 psi; Clutch cut-out 435 to 475 psi, clutch re-engages at 220 to 280 psi.
white casing switch 8L8Z-19D594-A - cooling fan ON 285-315 clutch cut out 435-475 clutch re-engage 220-280
gray casing switch 7S4Z-19D594-AA cooling fan ON 227-257 clutch cut out 420-460 clutch re-engage 220-280
If you go with the white casing switch, you can get the high speed cooling fan on sooner. I could not find a 4 pin switch with a higher cut-out pressure.
You can verify it is cutting out due to high pressure by hooking up a set of a/c gauges and watch as pressure rises on the high side gauge, and watch when the compressor clutch kicks off. If you hook up a jumper with switch to the 2 pins for the cooling fan, you can control high speed fan to go on when you flip the switch.
When I forced the high speed fan on myself, the high side pressure dropped immediately and the a/c clutch stayed engaged. I did this on my Mercury with similar sysytem. I think we delayed the high fan so we could pass the alternator charging system performance test. My preference is to have the fan come on around 200 psi. My Mercury switch is 4 pin, very similar, and the obsolete switch I installed turns the fan on much sooner. Not sure if it will physically fit the GT, but the switching is the same. Cut-out switch is normally closed, the cooling fan swithch is normally open. I can provide that switch part number and a source if anyone wants to try it. About $45. Gray casing.
Gerry from Ford Service Engineering