Growing up on dirt bikes, I have always valued light weight and maneuverability over raw power.
Have you considered the CBR300R? Might be a big enough improvement, yet familiar enough, to justify another purchase.
Yes, I've looked at the CBR300 but I haven't ridden one yet to find out just how noticeable the extra power would be. I nearly went for one that someone on Autotrader was very desperate to sell a couple of months back but made excuses to myself about the distance and aftermarket can.
I'm going to sound really picky here but one thing putting me off the CBR300 is the orange backlight to the speedo. I think it looks awful and dated compared to the blue on the 250.
My current favourite is the R3. I rode one when they first came out and remember thinking it felt pretty similar to my CBR250R. The demo bike only had a very few miles on the clock and I respected the dealer's request not to rev it past 7000 so I never got to feel the extra power over my 250. The ability of a twin to run smoothly at lower revs would be welcome.