Drew, I agree with Dan that the Kawasaki top box is smaller than the Givi and is about 4"-5" further back than would be a true backrest on a custom pillion seat. However, I think it depends on what you're looking for . . .
My wife, Denise, has her own bike, but also enjoys riding with me sometimes. We took the new Versys out, this past Saturday, for the first time with her as the passenger. It has a Kawasaki top box. When we got back after about 120 miles and a variety of road surfaces, speeds, curves, hills, etc., Denise said, "That's the most comfortable bike, for me as a passenger, that we've ever had." She liked the top box because it allowed her to relax a bit against it, once in a while, and it removed any fears (whether rational or not!) about sliding off the back during acceleration or flicking through the twisties. I've had a seat with a backrest on my last three two-person bikes (Harley Heritage Softail, Honda VFR800 VTEC [with a Corbin custom seat] and Kawasaki Vulcan 900 LT), so she was comparing the Versys's ride to a pretty supportive and well-saddled group of bikes! It's important to note that Denise is an "active" passenger who's in good shape, works with me to keep the bike responsive and keeps her weight (120 pounds in full gear) well balanced, stable and predictable. I doubt that the top box would work if she was a "passive," slumpy and/or inattentive passenger.
So, the Kaw top box obviously wouldn't work if you're looking for something to serve as the backrest for a "queen seat" that give luxurious support during long distance touring for someone who prefers watching movies on an IPod to watching the road and the traffic. For shorter runs and a ride-committed passenger, however, it got the "Princess Denise" Seal of Approval!
You might talk with your usual passenger about his/her expectations for support, preferred riding style and posture, etc., and then consider the Kaw top box as a viable option. One thing is for sure: it is a whole lot cheaper than a Corbin seat! [And I mean no insult to Corbin, which produces great seats that are a true value for the money you pay. But if you don't need one or want one, once they become available, then why not try a less expensive alternative first?]
Good luck with your pillion-seating search!