I've experimented, as I'm way too ADD to read up on it. Just my opinion, but:

700x18's and 700x20's tend to ride hard, and let's say my criterium attempt with them was not a fun thing.

27 to 700c conversions may change the handling, but you're dealing with the sum of three changes (size, tires, wheels). All are appreciably better quality than the average 27. For example, going from Rigida 27 steel on Maillard hubs/old Conti tires to 700c alloys on Shimano hubs and Michelin Citi tires was a nice change, but changing back to new Alex 27's with Gatorskins was a big step up from that.

I've learned from mrmw and miamijim how wheelsets make a big difference, and it's the combination of size, wheelset, tires that seems to be the improvement. I doubt I could make a less general statement than that.