As a tall rider, 6'4" and nearing 200LBS full gear, I had wanted to reply this thread, but guess my reply never got posted.
I don't think you need to worry about the performance of the CBR300R/CB300F bikes, as they can go at any speed limit, even with a 20MPH headwind it still can reach upto 80MPH.
I've gone as fast as 90MPH (GPS) on a stock bike; and more to go with mods.
If your reasoning is to commute, or joy ride (cruise), you'll not outgrow the 300s.
If your reasoning is to fun ride on the fast lane; yes, you can outgrow this bike fast (I already hit the limits of the bike on the interstate after 2 days), but will probably be able to squeeze out 5% more performance, and will be satisfied with that.
One issue I'm having as a taller rider (6'4"), just like mentioned above, is neck pains on the CBR300R.
Actually, I have it on most sports bikes.
I think it has to do with my larger torso, and in comparison to a shorter person, shorter arms.
Taller people lean forward more, and need to tilt their heads more upwards.
I'm not complaining about wrist pains, like other riders, since I'm not death-gripping the bike; but I do have a chronic neck pain from always looking up.
For me it can start from as soon as 15 minutes riding. Especially in the city.
Something what you'll experience less on the 300F.
Since you're not going to pin the bike wide open and hooligan ride it, I think the CB300F should be fast enough.
About the fuel, most people get into the 60's of MPG. Wide open I've noted as low as 37MPG. My first fuelup was 50MPG (breaking in the bike); but I think it should get better as I ride the bike more.
One thing to take into account after buying your bike, you need a helmet, and preferably boots, and protective gear (gloves, jacket, pants).
You also have to take into consideration that Honda charges for the first maintenance after buying the bike.
Unless you do it yourself, they charge just under $300 over here; and the time of the first maintenance depends on how much you have ridden it.
I did my first maintenance (oil change) on the second day of buying it.
If you plan on going for the 300R, know that the fairing is in the way of the oil filter, and is a bit harder to switch filter.
The 300F has direct access to the filter compartment.
Honda recommends 10W40 oil, but if you buy your bike in the winter, 10W30 will do as well.
Especially during the break in, if you're not over revving the bike.
I used simple car oil, and haven't had any clutch slippage yet, but don't count on that.
Anyway, much to say, so fire away those questions!