Does the game even let you install the EP's in different places from the main game? (without using simlinks as a workaround).
Assuming that's all fine, it would have limited impact. Anything that is a part of the base game would be loaded faster, anything a part of the HDD (the EQ's) would load slower. With all the EP I think the base game is a little less than half the content, or maybe a little more, so there would definitely be some expected improvement. If you wanted something faster than a hard drive, but don't want it all on the SSD, then you can compromise with a flash drive (memory stick). It's faster than a hard drive (maybe 2 or 3 times as fast), and you can get a 32GB without paying too much.