Is the performance of Lion + SSD + Filevault2 good enough for a system primarily used for application developemnt?
As you will buy one of the latest Macs, you should be fine.

The used Intel processors support Intel® Advanced Encryption Standard (AES) which is also used in Filevault 2.

Ars Technica, published an extended review on Filvault 2 in Lion. They say:


  Apple also leverages the special-purpose AES instructions and hardware on Intel's newest CPUs, further reducing the CPU overhead. The end result is that regular users will be hard-pressed to notice any reduction in performance with encryption enabled.


Edit:

These are real world numbers from my MBP with an Intel 2635QM processor which supports AES encryption. My hard drive is a Crucial M4 128GB.


Filevault 2 disabled: max. read 510MB/s, write: I forgot to write this down :(
Filevault 2 enabled: max. read 490MB/s, write: 190MB/s


I mention only the max. numbers as these figures depend on different factors for file operations. The difference for maximum read here is 20MB/s abosolutely which is 4% relatively.