I did a trip from Trenton ontario to truro nova scotia, 15-16 hours straight across 4 provinces. It was not a problem in my micra S and the little micra had no problems keeping up to the 120-140 average speeds through the 401 and quebec. I even drove on the highway when the tropical storm hit with huge winds and 100kph wind gusts.
They do get knocked around a little bit in the wind, but no different then any small, high car in its class and I didnt feel scared or leave a lane. But you could defintially feel it.
The only "mod" I did was that I put an armrest into the car by putting a squishy pillow between the front seats. Mine is a micra S and doesnt have the factory one.
You wont have a problem going to florida. My trip was 1400km in one shot. 2.5 tanks of fuel. It was a breeze.. I was a bit uncomfortable at the very end of the trip, but I wasnt sore at all.. Im sure Metro could pipe in, but the best thing you could do for a long trip is a beaded seat cover. Keeps the heat off you and keeps you nice and dry.
View my fuel log 2015 Micra S automatic: 7.9 L/100 km ... 35.7 mpg (Imp) ... 12.6 km/L ... 29.7 mpg (US) ...