The recirc panels in front of the driver and passengers feet are just a few pop rivets, 15 minutes total, do it
the fan is not undersized, in fact it is huge. The AC system is very 'lossy'. The worst part is the diverter assembly above the passengers knees. It seems to lose as much as half the air pumped out by the blower.
Look at my post, which adds to a bunch of work done by others:
One of the small things I did was permanently prop open the floor vent, which can be done with the nose on.
I notice you have a 'non standard' vent hose, make sure it is tight.
Of course it is possible that your blower is dying, or that the resistor assembly is getting fouled up. If you do end up getting into the heater assembly, do research all the things that are possible. My heat is hot in the winter and my AC has been tested comfortable in the high 90's, with 'essentially' stock components.
So the engine idle just drops too much with the AC on?
The only weak point on my AC is it stinks at low rpm. I have not gotten a lot of feedback from others about idle performance, probably because so many have AC that barely works at all. It is possible my compressor is weak, but I am thinking it might be the style.
The Elise/Exige has a scroll compressor, not sure why, it is a bit unusual. Could be it can stand high rpm, could be it was cheap.....