Manifolds vs. Headers

Just by the pic , I'd say your car is sitting at least 1 1/2 -2" higher than mine , that is the reason you might not be hitting as often as I did and others do . I really like carving up the back roads , so sitting up another 1 1/2 - 2' would hurt . Not saying any thing wrong with your set up , just different .
You would be surprised how dropping an inch will impact the headers , if you have 4" clearance now 3" is not much , that's hitting speed bump territory here , some here are over 3" high and 16"-20" wide . Always fun to see a Lamborghini cross them bumps .

your dart looks tuff in your avatar. i did put a couple of cranks on the torsion bars when i put the V8 in it just to make sure it wouldnt be bottoming out. i am usually on the backroads around here , so i dont encounter many speed bumps. so far so good, i dont think i will ever lower the front much if any. i see fools all the time ripping out the bottom of their acuras that are 1 1/2 inches off the ground lol