Dan, this is subject of great interest for those of us in the southern states. The biggest obstacle to getting better cooling is the available space under the dash doesn't allow a larger evaporator to be installed. I had a look at putting an E24 evaporator in, and unless modifications (total replacement) to the heating ducts are made, there isn't room. Early German AC was not very good with our hot climate even when new. Add to that the change away from R-12 and it's harder to make the coupe cold.
Much has been written about AC, in fact, Rob Siegel is putting the finishing touches on his latest book: The Hack Mechanic guide to vintage air conditioning. After a bit more review, he'll announce the release here I'm sure. I plan to buy it as I'm going to installing AC into my coupe for the first time when I put it back together.
From my research, there are several things that can be done to aid the cooling of your coupe. Put in a heater shut off valve. That way you don't have hot water under the dash at all times to fight against. Add good insulation to the floors and doors. There is lizardskin spray on stuff, I've got some of the stick on kind I'll be using. Don't forget the roof if you have the headliner out. Get the windows aligned so you don't have as much infiltration there. Use as large a condenser in the front as you can, I believe 19x22 is the agreed upon biggest size that's practical to put in. Thin pusher fans tend to make noise while older thick bladed fans are quieter and should push more air. Check the specs. There is lots of room in the front of a coupe, unlike modern cars, thus the trend to thin fans. Putting in a 'rotory' style Sanden style compressor is also helpful. No big pulses like the old style compressor mean no muffler in the system, and the compressors are smaller to boot. Since R-12 can still be purchased on ebay and craig's list, some choose to continue to use it, even though shops can't sell it. I think I'll use the newer, cheaper coolant. Some say it can get as cold as R-12, others disagree.
If you want to sacrifice your glove box, you could probably put a modern 3rd party system under the dash, but I choose to hunt and finally found the stock evaporator from a forum member. I had the AC grill in the trunk, fortunately as they are scarce. Markos is 3D printing them in nylon, I think. Yes, you can put a huge evaporator in the trunk and bring the cold air in via the parcel shelf and maybe under the back seat, but that's not really very clean.
I'm sure I've forgotten some details. but that's the basic picture.
Ian
