It's really a preference.. if you used 181 foils at -2, the centered 178s will feel shorter if that is what you are asking. The way it feels riding is that 181s at -2 feel the same (pretty much) as 185s at centre (in honesty i had the experience with 172s and then 176s but it'll be the same). If I were you I would aim for 183-185 and there are a few options in that size now.
182 Atomic Punx.
185 Volkl Wall.
186 Moment Jib.
That said Lines tend to come up a bit long, in terms of effective edge and you won't hate 178s its only a few cm less tip and tail.. I just think you might be happier on the longer skis.