That's the trouble. I'm only on a single core P4, so I didn't expect too much improvement from SP1. But I certainly didn't expect it to make the blurries worse! But looking around the internet, it seems that even the lucky folks with the dual cores are getting more blurries too.
I've seen this sort of thing before (with graphics card drivers) where developers craftily lower the detail (or the LOD radius in the case of FSX SP1) to get the numbers up. They seem to think that people won't notice. Well, they'd be hard pressed NOT to notice if they're using photoscenery!
I've tried various combinations of Hyperthreading on and off, SP1 installed and not, and even SP1 with the CPU affinity set for dual cores. (BTW, I know this is not intended for hyperthreading only CPUs, but it has been recommended on some boards and it doesn't hurt to try). SP1 either introduces stutters, blurries or both. It's off my system for the time being!