And the verdict:
The overall mechanical feel of the installed rack is good and solid. I was worried that it might feel wobbly or cheap, but once secured, the undertail handle frame is absolutely rock-solid and while there is a tiny bit of natural up-and-down flex to those long side arms with the case installed, it's nothing worrisome.
An illustration from my first local test run of the case. I found that it holds my XL-sized Shoei RF-1100 and Bags-Connection tank bag with, admittedly, an occasional small bit of difficulty in closing and latching. The latching system for the V46 is a little bit tricky to operate when the case is really full with helmet and bigger items; I probably need to take the document holder out. That should help. Locking the case to the platform is very solid and secure, with a "CLACK" loud enough to echo when I put the case on the rack in the morning before work.
Further along on my 135-mile test run. This is final config with inverted DOT reflectors installed, and some black and red Reflexite tape highlights added to the rack (as well as other parts of the bike).
About the Reflexite; I was concerned that the tail box and rack might obscure the tiny taillight on the bike from higher angles, so I added Reflexite SOLAS tape under the V46's transparencies (per a suggestion at ADVRider.com), and colored Reflexite V92 in other places.
The next day I went on a 420-mile round-trip from Dallas/Fort Worth to Austin and back, and then on the day following that I commuted to work carrying my (full) coffee pot, spare shoes and work clothes, breakfast, lunch, phone, brake disc lock, and other small items. In my opinion the rack and V46 have no effect on bike handling. If anything, it feels a little more stable in windy conditions, and at worst I think I might have lost a couple of miles an hour in top speed. I can't really speak for gas mileage, because I was fighting headwinds on the way down to Austin, and on the way back I was too knackered to pay attention. I'm in agreement with the notion that the Ninja 250's speed limitation is mostly aerodynamic in nature; my bike tops out at about 88-90 on the speedo no matter who rides it. The box itself is exactly as wide as my body on the bike, so it's probably mostly in wind shadow.
For me, this is a great mod which makes an otherwise useless "learner" bike into something real-world practical. Whether it's worth the expense and trouble is up to you. A lot of people have a real negative reaction to investing anything in a Ninja 250, but if Givi would bring this rack to the U.S. at domestic market prices, I think that might change, especially since Givi luggage can be transferred from bike to bike.