Focusing Sunlight with Flat Mirrors- solar barbecue Say somebody had 48 1sqft glass mirrors (almost 4.5 sq.m total), because these tiles cost about $1 each.
If mirrors are all tediously aligned to redirect sunlight onto a Weber BBQ, the air temp inside the BBQ quickly rises to 260F (400K), and the irradiated surface (interior caked with years of BBQ crud) smokes profusely.  
The question for y'all is: 
How complex of a model is needed to explain the current findings?
Could the system by upgraded/expanded to actually BBQ- air temps near 574F (also K)? 
Should this hypothetical buffoon pointing mirrors at his BBQ: a)buy waaaay more mirrors, b)try to improve the efficiency of the receiver (the Weber)?
Should I include more data (ambient air temp, irradiated surface temperature)?
I'm hoping I can turn solar BBQ'ing into a 5 person game before summer rolls around.   
 A: 
Could the system by upgraded/expanded to actually BBQ- air temps near
  574F

The energy in sunlight is about 1000 W/m^2. 48 1-foot tiles gives you a total of ~4.5 square meters. Now since at least some of those mirrors have to be tilted relative to the source in order to tile properly, there's a loss there - it's the same reason its colder in the winter.
So depending on your layout I'd say you might get anywhere from 3000 to 4000 watts onto the BBQ. I'd also think at first glance that you'll likely get the vast majority of it INTO the BBQ, as opposed to reflecting off. So the total energy is going to be maybe around 3000 watts.
That's the same wattage as the large burner on a typical electric stove. IIRC, they give off about 12,000 BTU. The old rule for a BBQ was 100 BTU per square inch, so in this case you would have about 120 square inches to work in if you want the same heat as a stovetop. That's 1-foot by 10-inches.
So it's enough heat for a steak, but definitely not enough to do any sort of major cook.
A: Let's ignore conduction, and pretend that the only way the BBQ gains/loses heat is by radiation.  If this bothers you, just build a transparent pressure housing around the BBQ and evacuate it.
Whether just sitting in the sunlight or at the focal point of your array of mirrors, the BBQ will reach an equilibrium temperature such that it radiates the same amount of energy as it absorbs.  Let's say sitting in the sun it reaches temperature $T_{sunlight}$, and you want to heat it up to a temperature of $T_{grill}$.  The power emitted from the BBQ as blackbody radiation is proportional to the fourth power of the temperature, so the area from which you need to collect solar radiation to achieve this is $A_{grill}\times(\frac{T_{grill}}{T_{sunlight}})^4$, where $A_{BBQ}$ is the projected area of the BBQ in a plane perpendicular to the sun's rays. 
As an example, let's take $T_{sunlight}=320K$ (I went a bit warm because of ignoring conductive heat loss to the air), and, as you stipulated, $T_{grill}=574K$. Then you need a solar collector that intercepts an area of the sun's rays $10.3$ times larger than your BBQ.  For a BBQ with a projected area of $0.25$ square meters, you should only need a collector with an area of about $2.6$ square meters. 
Since you will inevitably experience some inefficiencies, doubling the collector area seems advisable. 
A: Considering the discussion to date- it's looking like I'll need both more mirrors and to improve the receiver.
It sounds like I'll need at 10-15 square meters of mirror to rival the "BTU rating" of a similar gas grill.
It sounds like the BBQ desperately needs to insulated on the backside.  Also, I should probably scuff up the gloss-black paint.  I'm having trouble interpreting the "gloss numbers" and "light reflected values" listed for paints... but the suns reflection off the BBQ is currently strong enough to hurt my eyes, so I bet there is substantial room for improvement here.  
