As Porsche technician and owner of a 1983 911 that has most of the possible A/C upgrades I can tell you that there is no way you are going to get an 86 911 to cool anything like a Boxster/996. A Boxster will cool even better than a 996 just because the cabin is so much smaller.
The first problem is the condenser. The one on the 911 is a good size but needs the engine for cooling and doesn't get enough air until about 3000 RPM.
At slow speed there just isn't enough air flow. If caught in traffic I will keep the revs up to about 2000 to increase the cooling... I really hate being hot.
The second problem is ventilation. I have upgraded vents in my car (about 80% increase over stock) and it blows like a hurricane... but as Pete said it is loud!
The upgraded vent is not louder, the car is louder.
I was driving 30-minute freeway trips yesterday in 100+ degree heat and I had to turn my fan and temp down because I was getting too cold. About 40 degrees out the center vent which is 10-15 lower than the factory spec for a 964. But when I got to the stop light to stop light stuff I lost about 50% of the cooling and had to go back to full cold settings.
The 964 and later cars all have a single huge condenser in the front of the car with a huge electric fan on it. So the cooling is constant regardless of road speed.
I can't imagine anyone going from one of these late cars to a pre 89 car and not complaining about the climate control... Which it doesn't have. The controls are..."Unique" for someone not accustomed to the car.
This doesn't even start to scratch the issue of the several thousands of dollars in A/C upgrades you will need or the fact that it steers like a truck in a parking garage or the fact that the clutch is stiff and the shifter is worse...
Ahhh... but there is nothing like the feeling of driving an air cooled torsion bar 911 and listening to your 15-year old daughter giggle with delight as the tach searches for red on the way up a freeway on ramp
(and lifting before reaching the posted legal speed limit) 