Harsh ride is more based on the shocks/struts than the springs.Just bought a used 05 TL 6speed and it's got some coil overs on it and rides too harsh for what I want. Was thinking the H&R sport springs or Eibach. I want something a little lower and just a little firmer than stock, I think. I've unfortunately never driven a stock TL but I'm assuming it's not the firmest ride. I want to keep a nice ride quality but on the sportier side. If there are other springs I should look at and also shocks to go with them maybe some KYB's or adjustable koni's.
If you have coil-overs and want to go with springs, you are going to have to get struts too. If you get Koni Sport struts, it has external dampening adjustment.