Here’s my rudimentary approach to SE30 aero. None of this is based on empirical evidence. As I said earlier, I have employed my butt dyno extensively.
I agree with your assumption that the E30 suspension was tuned for high speed stability. The SE30 springs are stiffer than OEM. My sense is that the SE30 front springs are more stiffer (forgive the bad grammar 
) than the rear springs, so the front is less sensitive to added downforce. In other words, you can load up the front with downforce and the car will handle OK if you keep the rear bar soft and compliant.
The purpose of an E30 air dam is to direct air flow to brake cooling, oil cooling, and around the sides. It will eliminate some lift, but not much. The bigger improvement comes from adding a splitter, which as the name implies, splits the airflow creating a clean, low pressure area of air that travels under the car. As long as the splitter is at the height of the oil pan, it is as low as you need. This coincides with the bottom of the factory “chin” spoiler on an 89-91 car.
Depending on the depth of the splitter tray in front of the air dam, you will get various levels of downforce. My splitter is built to the extent of the rules (2" out from the bumper outline) and is almost cartoonishly large. Based on Traqmate data at Miller, I could not discern any significant difference in segment times that included large straights. I attributed that to the assumption that the drag slowed me down, but I was getting on and off the straight faster due to better grip. In the twisty parts, there was not a clear time advantage with the splitter but I was driving more cautiously with the splitter since I didn’t know how it would affect the car. Essentially, I was able to drive more comfortably at the same lap time. I pushed harder as the week wore on, but never went back to back-to-back testing with and without the splitter.
The other advantage of a splitter is the flat tray behind the air dam, which helps smooth airflow. As Haynie said, that’s probably a moot point with a flying brick. But in theory I feel quicker.
A taller air dam (i.e. taller than stock) will not aid handling in my opinion. Once the air dam is low enough to split the airflow - even only modestly - and provides ducting for all the cooling demands it has done its work. When air hits the spoiler, it tumbles up, down and around the sides. The splitter prevents the tumbling down, hence its advantage. If you cause even more air to tumble, you will negatively affect drag without increasing grip. Is bad.
Regarding splitters and offs… Splitters are like tires and brakes. They are consumables. Bring a spare.
That being said, I don’t plan to run a splitter at the typical Southeast track. Splitters help eliminate push most in medium- and high-speed corners that are NOT preceded by a braking zone to set the nose and rotate the car. It seems like you can remedy push at the Southeast tracks through technique, tire pressure, toe, and cross. None of those are as drag-costly as a splitter.
These are all just opinions. Flame away.
Steve D.
