I run Parallels for Win7 64-bit on my MBAs. My older MBA, a 11"/4GB/128GB SSD, always impressed me how it performed very much like my rather powerful PC for the tasks I do. I am a tech translator and have to run Excel, Word and Powerpoint 2010 concurrently, a dozen PDF windows open in Reader and a somehow heavy translation memory manager app.
But I always wondered if doubling the virtual environment RAM (4GB for each OS) and spicing up the CPU would make things way faster (despite my job tasks do not demand so much speed, but a lot of memory, virtual and non-virtual, when reindexing DBs...) and I upgraded to an MBA 11"/i7/8GB/256SSD. Funny enough, the ONLY difference I really noticed it was, in fact, that 256GB SSD "slowed" things a little!!!! Although the newer system pages much less (RAM to SSD), I cannot feel that in normal work...
Coming from a PC background, I am amazed at how efficient is OSX in managing hardware resources. Bottom line: I wouldn't notice the diff between my older MBA and the new, maxed out machine, in terms of performance (for the tasks I run, of course).
Edric