I can't believe this is true, but I switched from 1/4" front spacers to 3 MM and my front end suddenly rides much better.
I got into the spacer game a few years ago when I put 17" 996 wheels on my car. These newer wheels require a spacer, but only a very thin one, because they won't sit all the way down onto the 968's hubs.
Anyway, the problem with these spacers is the weight of the car and the road forces are apparently carried only by the five wheel bolts. Wider spacers with a raised hub ("hub-centric") carry and transfer these loads onto the hub, and thereby preserve the original design. Problem is, very thin spacers don't have enough material to be hub-centric. I know cause I've looked for them and asked suppliers for the past several years and they just don't exist.
I actually thought about having the 996 wheels machined to match my hubs, but didn't want to go to such an effort. It would be interesting to try though, to see if any further ride improvement would result.