Okay, so I promised I would do a write up on both of these.
H&R Sport Springs
So, the general consensus in the community is that the H&R Sport Springs are, with regard to street use, pretty much aesthetic only enhancement i.e. moar low. I would agree with this with regard to
street use. On the street, car rides like stock, and the only thing I notice is that my car 1. looks way cooler (again, moar low) and 2. scrapes when I go over a pop-tart.
However, it's a very different story on the track. The springs definitely add a noticeable amount of composure when you're doing a buck twenty around turn eight at Willow Springs. If anyone has seen Randy Pobst's review of the M235i, he mentions that the car has too many "secondary motions" on the track (in stock form). The H&R springs help deal with that. Car feels much more composed/stable at track speeds far in excess of what you would or should ever be doing on a public road. I am a big, big fan of the track performance of the springs. Good on you H&R.
Schroth Profi II 6-Point Harness
Thread on my harness install --
http://www.2addicts.com/forums/showthread.php?t=1110227
Okay, seriously, if you plan on tracking your car a lot, GET A HARNESS. Makes a HUGE difference on the track. You can actually concentrate on driving rather than trying not to fall out of your car. Great, great investment for those of you who are keeping this car for a long time and plan on doing significant track duty. I highly recommend it, and especially the Schroth belts -- extremely easy to adjust very high build quality. Top notch product.
One final piece remains -- my LSD (which should be ready in two weeks). Super excited. I think it should be the last major upgrade I'll need to do for a while. Car should be a track weapon with a proper rear end (sort of already is, but the lack of an LSD does hold it back compared to other cars at the track, I think).
Any questions, leave them here.