Curious on this, myself. I got rid of my 88 and the rest of my mustangs are earlier models with the sealed beam headlights. But my buddy (owns my old 88) is wanting to try led's. I can tell you that I purchased housings to replace the sealed beams in my 79 and then installed Beamtech LED's. The brightness is several times better, now... both on low and high beam. No complaints from incoming traffic (as long as I'm not high-beaming them). I couldn't be happier with them. We purchased Beamtechs for the wife's 2015 4Runner and while the high beam is super bright, the lows are pitiful. We've yet to look into it, so I'm not sure if it's due to the housing/reflectors or the bulbs themselves. Highs and lows take different bulbs.
I think there's just some trial and error required to get bulbs that work well with your housings. Surely, there have been 87-93 mustang owners who have figured it out. Unfortunately, there are several different companies making different styles. The LED technology is constantly changing, so it's hard to get the latest info.