They are coming on well. I have done the same trick for weight in plastic kits in the past, filling toilet compartments with lead and even plasticene at times!
Just an idea but for those seats that show in the windows, is it possible for you to get a paint brush on to the very edges without touching the glazing itself? If you can, you could paint the visibe edges with black to disguise them somewhat.
p.s. I agree with the comments re the Bogie 'B': i have frequently criticised it for the over-complication, using 100 parts where 30 would have done (so to speak ... don't quote me on those numbers!). And, after all that, they still didn't include the window grilles! It still made into a nice model in the end, though.