Well, the impact on search query length may be negligible but there are other ways in which Instant may be changing search behavior. I have a small slice of data I hope to post in the very near future.
It's not about the query behavior, but about assessment behavior. As such, your mileage may vary based on your particular rank for terms.
Outside of this analysis, the keyword length could remain the same, but a site may receive less traffic if the auto complete algorithm has changed for critical terms. Again, this will vary site by site - and where one loses, another will gain.