Will lip need balanced in rear?
 

1.6L HPDE/Dual duty. Engine w/ I/H/E, MS2 & E85. Xidas w/ 700/400, RS-4 195s on 7". Still learning but getting faster each time. As a 1.6, every bit of help counts. I'm considering putting a GV lip on, more to reduce drag than for lift reduction, though I know it will reduce lift.

Will it alter balance enough that I'll need to add a duckbill, etc., at the rear or just tweak suspension to rebalance? Any guesses? I'd like to avoid making the car too boi-racer.

No. I doubt you'll even notice the difference between lip/no-lip.

All I have is a GV lip....I put it on 3 track days ago...I haven't noticed any positive benefit or caused any downside (other then clearance when I load it on the trailer!). Was very neutral in the corners before and still is.

I will probably do a bumper cut next before doing any more aero...

You will never notice it.

Look at the setup every Spec Miata in the country uses.

And don't cut the bumper, looks stupid and doesn't do anything.

You won't notice a difference. The only thing that may be measurable is a decrease in drag, but any decrease in lift wouldn't be as forward biased as you're assuming. Also, nothing wrong with a bumper cut, there's a lot of evidence, data, and testing that the bumper produces drag.

Lip by itself wont do much, but if you combine it with a plywood mushroom shaped undertray, you can get noticeable downforce improvement.