If you're concerned with ride height and harshness, I would go with the Racing Beat springs. I have them on my P5. They made a huge improvement in handling and they still don't wake up my 3 year old daughter when she's sleeping. They're very comfortable and the drop was only about an inch.
I'm running Falken ZE-512's 205/50/16 on stock rims. So far they're pretty good all around/all season tires. I'll go with something more agressive when I throw on the 18" BBS RG-R's.
03 P5 5spd