Lovely work, gonna see about sending you some PR goodies that might work in the map
About the overgrowth- I've had some trouble with the editor crashing when lots of differnt types of under/overgrowth were used (so did a few other people), so the best thing is to initially paint say just 2 types (maybe a trees/bushes one, & another for just bushes), & for each type just have 2 or 3 objects.
Don't go placing single static overgrowth unless it's vital- keep things easily editable, & you can use the overgrowth shadow map on the overgrowth system objects.
I like the way you're clustering the veg, very authentic, try to be economical with the density though.
Only comments about these screenshots are the big highway bridge seems a bit redundant leading up to a few small houses, & the huge city walls surrounding the mosque look too big, although it does give it a Mecca look.