What's the difference?

The difference is that R-12 (also called Freon by it's DuPont product name, is a chloro-flouro-carbon (CFC) that when released into the atmosphere will deplete the Earths ozone layer, which protects all life on Earth from excessive Ultra-Violet light.

Everything I've learned points to a temperature increase of only 4 or 5 degrees up to a max of 10 degrees Fahrenheit at the vent when switching over to R-134a

Here's some of what I've learned about doing A/C conversions to R-134a:

1. The seals must be replaced with green colored seals made for R-134a.
2. The original mineral oil lubricant must be drained, preferably flushed out (varies by reference), and an Ester based oil (compatible with R-134a) installed.
3. An R-134a compatible Orifice Valve should be installed.
4. The Accumulator should be replaced.
5. The fill adapters must be replaced to fit R-134a fill equipment. See notes.
6. An R-134a sticker label must be attached to inform other service techs that it's been converted. See notes.
7. Sometimes a larger Condenser may be recommended for extremely hot climes (such as yours), and also a "pusher fan" mounted in front of the condenser.
8. All references say to use less R-134a than you would need R-12 because it's specific gravity is lower. The references vary from 10% less to 20% less or conversely, use 80% to 90% of R-134a vs. R-12. See notes.
9. All references caution that over filling the system will cause an increase in vent outlet temperature. See notes.

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Notes:

5 & 6 Required by the EPA.

8 & 9 From an evacuated state, follow this procedure (especially if you don't know what pressures to use:

1. Insert an A/C thermometer in the center vent of your dash.
2. Attach LOW and HIGH pressure gauges
3. Turn on the A/C to Max and open the car doors. This prevents the A/C from cycling.
4. Add Refrigerant to an amount that would be 75% of what it would be with R-12.
5. Slowly add more to achieve the lowest temperature at the vent. If the temperature starts to drop, remove some of the refrigerant to bring the temp down to it's lowest point.
6. Record the LOW and HIGH pressures for future reference.

If you think your system may be over-charged, follow this same procedure for Steps 1 through 3. At Step 4, recover some of the refrigerant to bring the vent temperature down to it's lowest. Then record the pressure values. Adjust as necessary.

If you don't live in an average climate. Most of us don't experience 100 + degrees F. for over two months in a row. I wouldn't recommend a larger condenser, but if following these guidelines doesn't help, you may want to consider a "pusher fan" in front of the condenser.