I've had the CBR125 and yes it went on the highway but I sure did NOT enjoy the experience all that much! Especially on longer trips...but that just made sure I took more back routes and enjoyed the scenery a bit more.
As for down grading, I guess it all comes down to what your real world uses are. I'm up to 24k now on the CBR250 and frankly I couldn't be happier. Every time I think of upgrading I just realize that the bike does 95% of what I want it to do...and I'm cheap! The bike is paid off...the insurance is $500/year...gas is 3.5-4l/100km...hell it's that good at what it does that I'm in no rush to buy a car right now. Although I'm quite sure I saw unL33T's car today on Eglinton/Renforth which looked sweet with those white rims!
The ONLY thing I would prefer is to have a bit more top end, and more space for a passenger. But outside of that no complaints.
I wish that the Honda 300/500/650 were around when I was looking at bikes because those would meet 100% of my needs. However for now, I'll just enjoy the bike until I feel like it's time to move on to something bigger, and when I'm really bored of riding it!