I knew you'd mention the trees.
Of course I'm going to leave them in place. However, in stretches that are bare of trees where shitters can get up to full speed, they still can't touch my turret banks. Maybe occasionally a behemoth spitter will get one shot in on one turret, but that's extremely rare. The thing is, the bigger an attack, the more damage flamethrower turrets are able to deal to the horde, so increasing the amount of shitters past a large wave doesn't help them or tax the defenses any further.
Anyway, I'm sure you can crank biter nests way up in the map generation settings if you so desire. As I've mentioned, I used the default generation settings. Personally, I'm not a big fan of spending all my time and resources beating back biters. They're annoying enough in the end game as-is, what with trains stopping dead if they hit behemoths (unless you add multiple engines, which is a logistical headache in itself), flamethrowers destroying your own rail signals and electrical poles during constant biter attacks unless you design your infrastructure very carefully, etc.
I'm fairly sure the developers toned down biter density significantly while decreasing the maximum distance they can build from your structures (it's almost literally spitting distance now) and vastly increasing the distance between resource patches. I have my save file from our .15 game, and resource patches in this new game of mine using the new mapgen are much, much, much further apart than they used to be. Resources and biters now also increase in richness the further you move from spawn without needing a mod for that purpose.
If I actually had to deal with massive carpets of biters with these new generation settings, I'd just build an astronomically huge wall instead of a merely titanic wall, and that would ultimately be much easier to deal with after the initial tedium of setup than leaving nests alive interspersed between outposts. When pollution is far enough away from nests, as is usually the case if you build an ultrawall, they barely attack at all except to try to found new nests.