Yeah, the lack of cruise is one of the annoyances I have with the C 400 GT, as I do, or try to do, one or two multi-day highway slogs each year (to visit a daughter in Asheville, for instance -- the good part of that is that Deals Gap, the Cherahola Skyway, etc., is then only two hours away).
I find that a tremendous oversight, given that the 400 has throttle-by-wire. How hard would it have been to add cruise? See my gallery pic https://billanddot.com/C400GT/#S-12 for my halfass solution, and another recommendation.
The other gripe I have regards being able to add an external TPMS sensor to only the rear wheel (and an internal TPMS sensor is not offered as an option). See this particular pic and caption for my comment on that: https://billanddot.com/C400GT/#S-61
Now, having mention those negatives (and the underseat storage is nothing to write home about, either, and I wouldn't kick an aftermarket suspension out of my garage), let me also add a positive, since you also mention this: the handling is superb!
At home, in a total of about 6,000 miles now on the two C 400 GTs, I have yet to scrape anything on a lean, and I've tried -- and have always felt very planted and confident while trying. This is a first for me, over the course of 26 years, covering an R850R standard, Valkyrie and Victory baggers, and two Burgman 650s (and, after only 2,000 miles, I'm still getting acquainted with my other current ride, a Meteor 350, but I expect to grind off its peg feelers this season).
Last July, I participated in Edelweiss' Ultimate Alps Tour ... on a C 400 X; see https://www.billanddot.com/Ultimate-Alps-Tour/ . We sorted ourselves out after a day or two, and I hung out in the fast half of the ten customer bikes (most of which were RTs, with couples aboard). Over the course of a week-and-a-half of hairpins, I got down to lean angles of 43° left and 41° right (I have a modern GoPro that can accurately track this sort of thing), which is pretty far. And only one time, and very lightly, did I manage to scrape anything (and that was on a tighty-righty, suspension-loading downhill switchback). My point is that I put that scooter through its handling paces, and it came out smelling like a rose.
And that's why I really like the C 400 GT, even with its shortcomings. I'm old, but not dead (or all that slow) yet. Well, that, and my wife told me to buy one.
I actually sort of expect the feedback with regards to the handling of the C400 GT. This is the advantage of a smaller displacement bike that weighs less than a bigger bike. While I have two liter sport bikes and do track time on an S1000RR, I've been kicking around the idea of getting a Ninja 400 or maybe a ZX-4RR for precisely what you mention with the fun factor. I want to be able to full hooligan on the streets in corners without being immediately thrown in jail. I know my RR will have me in prison without leaving first gear. Same with my old ZX-10R.
One question I do have about the C400 GT, how are the brakes? That's one of my gripes about my C650 Sport. I don't care for the lack of bite and feel of the front brakes. This shouldn't be with dual rotors/dual caliper front brakes.