The tech's at max motorsports told me it would be OK and not too harsh for a daily driver - but also that some of the guys in the shop disagreed lol... So I went with urethane...and urethane is PRETTY damn stiff!
Coil overs would make delrin bushings much more tolerable on the streets. With the spring more inboard as the stock setup, it loads the control arm bushing hard and really causes a lot of extra harshness. Coil overs move the spring outward and take the load off the control arms and actually allow the suspension to work.
Actually, just skip the bushing and only do coil overs. It's the single biggest gain you'll make in handleing. You can get higher wheel rates AND improve harshness.
I ran Delrin on a DD for years, though it was not a Mustang. My take in general (take this for what little it's worth) is that the ride with Delrin really isn't significantly stiffer than PU, but it does remove the deflection which still affects PU. I love twisties and LOVED the Delrin if the roadway was decent.
I think PU and Delrin both suck on heaved, rutted or otherwise chunky roadways however. It doesn't sound like Delrin is a hot ticket for what you're going to be doing.
IMHO, they are great on roads where bumps are minimal. If you have a lot of bumps, IMHO they can actually hurt performance a little (I always felt like the front was chattering over irregularities because there was no absorption via the bushings). Note that I had them on a corner carver, not a straight-line car.
If you haven't, I'd do a search or thread on Corner Carvers. It was fortunate that Bhuff30 had some experience with Delrin on a Fox chassis (which I myself do not).