Well, I'll jump in whilst we await Steve's response.
One difference is the compressor. While all the 124 cars use Nippondenso, the earlier cars use lower capacity compressors than the later cars. The early cars use the 10p15 or 10pa15 compressors, while the later cars switched to the 10p17/10pa17 compressor. About 15% more displacement, definately better performance at idle and low engine speeds.
Other changes - the expansion valve is calibrated a bit differently for R-134a vs. R-12, though this is a minor issue.
Whether the condenser and/or evaporator were changed I don't know. As you've noticed, the cars which came with R-134a from the factory had dual electric fans, presumably to increase airflow across the condenser.
It's tough to believe your car is inadequate that far up North. I'm driving an '87TD wagon in Dallas, and while the a/c is not overly powerful, I get by just fine. A bit more cooling around town would be nice, but it's fine on the highway. I recently measured 47F discharge air on a day when the ambient temp was just a hair shy of 100F.
Here's some ideas for improved performance. I'm making these up as I go, so be gentle:
1) Make sure the fresh/recirculate doors are operating correctly.
2) Tint the windows - I've had good results with metallized tint, really cuts down on the radiant heat.
3) Make certain the correct auxiliary fan swith (on the dryer) is installed. Red color switches on the fan sooner than green.
4) Remove the resistor which runs the aux fan at low speed, run it at high speed when switched on by the a/c.
5) Have the R-134a charge checked - a bit low and cooling is suddenly no good.