If you want to go lower than you already are, then definately don't go 'drop in' bags, with the mounts they have on them it makes it like 1/2" to an inch higher than bolt in jobbies.
I'm guessing with out getting 'funky' - with some custom control arms, you won't be getting much lower than the spring out, sitting on the metal hump (bit that the rubber bumpstops to sit over). Even though you cut them out, with the bag inplace it'd be similar height? (Maybe a lil lower?)
Although, you could recess them into the control arm a bit, but I'm not sure on the legalites with chopping them up...? Or even recess them into the floor a bit!
If you know an engineer, pick his brains!
Not sure with where you are, but engineering bags can be pretty crap. Gotta still have 100mm ground clearance, with bumpstops in. Self-leveling in drive? To a programmed height? Not being able to 'use them while driving? Etc... Accuair, Dakota digital... Or If you're just going to run them til you get picked up on them, then go for it
Speak to DW Customs for some info, they'll be able to help you with all the legal side of it too. Theres a few other companies doing them, but I'd steer clear of cheap crap.
And, a bit off the topic of bags, is this manifold you're getting similar/the same one you showed me in c/b? If so, ####in sweet as!! Gunna be even moaarrr of a beast