Jimmy, the number on the vent is impressive. I've used it in an old Saab when I lived in Atlanta and it worked so well, I got closer to 30 degrees using a meat thermometer like the one in the picture.
However, I can say that R134 can also work quite well, and will ultimately be easier to maintain for someone who's an A/C novice. I have R134 in my 6.9 and it's ice cold from the moment the car runs.
I think that my issue with freeze 12 is not that it's an alternative to R12; but that there is a misconception that it is a
drop in replacement to R12. It's simply is not and anyone who takes the time to read the literature will see this. You do need to change the oil, and you will have leaks unless you change the hoses to barrier hoses so in effect your doing a conversion which may or may not require new parts. If it does, then you convert with the understanding that pretty much anything you buy will state something like "Warranty only valid if R12 or R134 is used".
The only person that touches my a/c system is me and I'm quite happy to take responsibility for my mistakes so I don't mind experimenting with certain things. I'm sure that you're the same. But for the people that don't maintain their own cars, they may want to maintain a warranty on new parts and be able to take the car to someone who will be able to work on it without trashing their own supply of refrigerant.
Therefore, fittings should be changed and a sticker should be added to indicate that the system is using freeze12.
Here is my 6.9 running R134. I'm using an infra red thermometer so the temps will show higher then yours because your taking the temp inside the vents.
When I took this picture this evening, I just cranked the car and the a/c setting is set to low. The other pic was taken outside the car to show the outside temp.
I bought this car more then two years ago and it was already converted. I've never had to top it up since. It has new hoses, aluminum ACC servo and a new condenser. I would consider it as a good conversion and it works quite well.
I've had cars that have not had good R134 conversions such as my 81 500SLC which I converted back to R12. The a/c worked horribly on that car when it used R134 but it works quite well on R12.
As we're taking about alternative refrigerants, a buddy of mine recently bought a 75 240D (W115) with non working A/C. The hoses were cracked and had to be replaced as well as the compressor (york) which was trashed. I offered to sell him R12 or give him what was left of a jug of Autofrost R406a. When I was looking for alternatives, this stuff was the only stuff that I could find that really was a drop in replacement to R12. By drop in, I'm
not suggesting that you mix the two but that it's compatible with mineral oil. On an inefficient system like the one on the 240D, I am amazed at how well it works. This stuff will turn your nipples blue! As soon as the compressor turns on, the low side hose near the fitting will ice up. I was so impressed, I bought another jug of this stuff to put in my W114 280CE.
I'm not against using alternatives (except for propane as I have alternative means of removing facial hair) but I do believe for the alternatives to work well, the system will need some work also. If you want to keep the system running at little cost or with little effort. R12 is still the most affordable option because you can buy cans for about $20 which is not much more then R134 (about $12 at O'Reilly's) and it will work as designed.