http://www.tweaktown.com/articles/1472/intel_p45_vs_x48_crossfire_performance/index.html
Tweaktown did a recent test between the P45 and X48 with 4850s and found that the 8x/8x was slower, by varying degrees, than the 16x/16x. Now, I take everything tweaktown says with a grain of salt, they aren't a great review site, but the test looked pretty well done. Check it out and see if the performance hit 8x gives is acceptable for you.