I know of one other leaf owner with a 2011 that has had a 100% grill blocked in the winter without trouble. I never got around to doing mine before the winter and since I haven't run into a problem making my round trip with a brand new 2015 I put it off to the spring.
As many people have touched on before the opening is designed for all conditions. Nissan has to make it big enough to cool a car going through death valley on the hottest day in July with 5 adults in it at 80 mph and stock PSI inflated tires with the AC going full blast. Once you factor in your unique conditions there is a lot of the grill most drivers can block off. Less energy needed to move the car down the road also means less load on the car which means less need for cooling. If you run max sidewall PSI LRR tires keep it under 60 mph and don't crank the AC then every single car grill opening out there is overkill for you. As far as the heat pump goes a grill block probably makes it more efficient allowing the ambient air under the hood to be warmer and instead of bleeding that heat to the outside air you can harvest it through the heat pump.
ICE drivers have been doing partial (75-90%) and full grill blocks for years without major issues. I ran a full block for winter and 80-90% for summer on my insight, prius, optima and miata (didn't run the winter with that one though). A good test is to wire up a little LED to the cars fan. If something gets hotter than the car likes then the car will turn the electric fan on in an attempt to aid cooling. At this point you see the LED go on which means you should probably open up the grill a bit.
I'm interested in trying the mud flaps as I removed mine the week I got the car. Not too crazy about the screws through the side though. I think some good 3M tape and screws into the inner liner should hold it.
Here are some fun tools I've used to find out what can be done with the leaf for aerodynamics.
http://ecomodder.com/forum/tool-aero-ro ... stance.php
I used the numbers from car and driver and the drag queen test, 3353 lbs, .32 cd and 24.5 frontal area. Engine and drivetrain efficiency and parastatic overhead are for MPG, they don't affect the rolling and aero HP. http://www.caranddriver.com/features/dr ... ecs-page-7
At 70 mph the calculator gets the same results as the car and driver test, 18 hp aero. At a more useful to us 60 mph it's 11.54. If we could get the car down to a prius like 0.26 at 60 it would be 9.38. or 18.7% less HP needed.
I used 0.01 for the rolling resistance based on this http://en.wikipedia.org/wiki/Rolling_resistance
. It's no surprise that LRR would give you much better results than aero for people who keep the speeds under 35 mph. A 20% decrease in rolling resistance as quoted by many tires would only give you about 6.3% less hp needed at 60 mph.