My understanding, from reading here:
http://www.ackits.com/ is that 134a conversions on some vehicles do not produce very good results. (They have a very good forum on AC questions.) They recommend retro-fitting to a parallel-flow conderser for better cooling. I've seen this mentioned by many AC gurus on this forum but have not read of someone that has changed to one of these condensers and reported their results. In Texas, R134a systems are marginal at best.
(What a p.i.a., this greenhouse gas idea is a lot of politics and very little science.)