Did you ever get around to blocking your grill?
With mine I just used a piece of garden edging, cut to fit and ziptied in place. I will try and get some pictures. (probably took about 5 minutes to do) I wanted something that would be easy to remove, just in case.
Basically, if you WD40 the plastic after you install it it ends up looking really shiny and for me, similar to my stock paint. It really isn't noticable at all, and as long as you use black zip ties noone will notice them either. (I only used one at each corner)
If I were you I would also look into some different wheels. The scion steelies, from what I have read, weigh in around 22lbs. While the Scion Spiral wheels that I have are around 14, and the 7 spokes are said to be about 16 or so. You can find the stock alloy wheels on ebay all day long for $150 for the set. While a lot of people on Scionlife.com and Clubxb.com like to run the steelies with "Beauty rings". Or if there are any "Honda-tuners" in your area, they like steelies for track use. (so it shouldn't be a problem selling them)
I eventually plan to install Enkei RPF1 wheels, which weigh 9lbs each.
I feel like the XB is just a lost hope as far as aerodynamics go and I try to make up for this by never driving over 60mph unless someone else is in the car. (usually on vacations) I was a little more of an overzealous hypermiler whenever I had my "VX Civic".
I really don't expect the XB to get anywhere near 58MPG average and I am completely happy with it getting 38mpg consistently and 42 on those random "good tanks"
Although, I have random thoughts almost daily of ripping the A/C, belts, Alternator, radio out of the car; installing a solar roof and a charger outlet to charge a row of Deep cycle batteries that would be installed in the trunk; lining the underbody with Coroplast and aluminum; removing my mirrors, windshield wipers; blocking off the front bumper completely; removing the spoiler and lining the entire rear of the car with VGs and or Airtabs; removing the carpet, carpet padding and rear seats; gutting the dash; removing the metal bumper supports in the front and rear; and replacing the roof and all windows on the car with plexiglass.
But then I drive the evil thoughts out of my head and realize that everything just might be OK, and if I work hard enough, maybe I can afford a Tesla Roadster one day.
Either way, good work!
Order some golf shoes, otherwise we might never make it out of this place alive.