Coming from the UK and understanding very clearly the issues of 'advertising' on forums, I am afraid I will have to refrain from mentioning any businesses in my post here.....
I would like to maintain my position as a technical factory resource for fitting/setup advice etc and avoid sales entirely. - I can do this as we already have a great BMW partner (some of you have already had contact with them) that will be able to answer your commercial questions. I hope you appreciate my stance on this.
Of course, when the time is right; I anticipate Nitron achieving 'advertiser' status on the forum.
As mentioned, I have an E46 M3 - which also means I have the rear subframe cracks! Infact, they have just started to appear following a heavy UK track day to compound this problem. I look forward to a weekend of welding reinforcements in January!
I run coilover rears with a relatively stiff spring rate (Track day rates), every day.
Rear coilovers -
Advice is really dependent on expected use. The 'issue' of running a coilover rear-setup without a rear roll-cage is of course that the turrets are not designed to cope with both damping and resultant spring forces. By considering both the spring rate used and (significantly) the weight of the vehicle we/you can judge what is appropriate. If you plan to ride 5-up, with luggage around the Nurburgring - then we would of course recommend that the car is either a non-coilover or the rear-turrets are reinforced - no matter which platform you have.
The E46 is known for having well constructed turrets (if not the subframe!), as is the case with the E9X. Nitron would not naturally recommend running stiff rear coil-overs on an E36 that has not been reinforced.
When running slicks - again, the advice would be to reinforce.
Having said all that; we know of many customers in Europe running coilover rear setups in M's without reinforcement or issue for many years.
A point worth making is that Nitron kits are all hand built to order and specification. As such we can create 'hybrid' kits that comprise of solid front top plates (camber/castor adjustable) with the rear as either a separate system or coilover. Further to that, the rear damper can be specified to either mate with the OE rubber top mounts (for mainly street driving) or with a full race bearing top mount. Both have their pros/cons, however I personally mated my rear shocks to a new set of rubber OE mounts. This results in mainting OE NVH and adding an extra level of compliance without sacrificing track performance dramatically. Of course, serious track cars and race cars will benefit from the spherical bearing mounts considerably.
As mentioned; our US BMW distributor is well worth contacting directly as they have a wealth of knowledge and have been running Nitron on their own cars for some time.
Hope that helps in some way?