razman, I've been enjoying what comes off my char griller with the sfb for several years. Personally, I've never worried about the smaller coals falling through the grate, the cross pieces are only about 1/2 inch apart. I add more coals every hour or so anyway to keep a consistant cooking temp. I also use a lot of chunk hardwood, in combination with coal... for the smoke, (rather than soaked chips) and heat.
I actually believe if you put a metal sheet with smaller guage holes, over the existing grate, you'll find that the ash will build up on that plate, rather than falling through the grate. I think this would be worse for your coals to set on, than letting the small pieces fall through to the bottom. I believe because of the location of the vent holes in the sfb (half above, and half below) the grate, and the location of the opening into the grill itself, (several inches above the grate) that even a moderate build up of ash, or smaller coals below the grate will not affect the performance, or ability to maintain proper heat. My grate - after so many years of heat, is actually dipped in the middle, leaving my charcoal and wood barely an inch above the bottom of the sfb, and it's still never a problem.
But... that's just my opinion based on my own experience with it... I'm sure you'll get many years of satisfaction out of it.
30" Char Griller Professional w/sfb
32" Stainless Steel Natural Gas Grand Hall w/rotisserie
37" Dual Chamber/ Front loading Brinkmann Professional
40" Stainless Steel Masterbuilt Electric Smokehouse