Hi,
Welcome, and I hope that your experience with the S80 is a good one. I have not done as many mods to my car as I would like, but I have done a few.

First, like you I hate the soft feel of the car, so I lowered it and firmed up the ride with Eibach. In retrospect I wish I had gone with IPD springs. I had IPD springs on my Volvo 850, and they were great in the handling department, but since they were linear, they were pretty stiff...even for me, I like my car stiff. My complaint with the Eibach springs is that I do not like how they behave with weight transfer from turn to turn -- they rock a bit -- but as soon as they load, they do hold quite nicely. I also helped out the handling by getting a set of IPD sway bars. These are probable the best investment I have made yet to my car. I can only say good things about them.

On the performance side, I have not done much. I got it chipped. As for whether or not I like the results, I will have to let you know later. My car was acting up -- the throttle was surging occasionally -- before I got chipped, but now it is worse. I do not know if it is worse from the chip, or if it just feels worse because there is a lot more power, so the surging feeling is exagerated.

As far as the transmission, I don't know. I do not have enough HP to have to worry about that yet. I did here from others though, that the Trans cannot go to much higher than 325HP before having problems.

Hope I have ben of help.
-J.P.