I can only give you a real-world informed opinion. If you aren't using the Greenspeed heating portion and/or don't have good installation, then no, the anything above 15 SEER isn't worth the additional money. You won't get anything close to 20 SEER without good insulation.
I would actually go with the Carrier Comfort series and insist that Carrier send you the soundblanket for free, Have your dealer install it under the guarantee. A soundblanket over the compressor is the main difference between the Performance and Comfort.
For additional comfort consider a whole house dehumidifier and/or a variable speed blower in the attic and maybe a nice filtration cabinet, like an electronic Aprilaire 5000 or a Lennox LC16. That's the most bang for the buck.
Q