Upgrading cars in Germany - TUV situation?

[MasterBlaster]

I have a 993 , registered in germany, and I am now looking at the usual upgrades ie bilstein shocks with lowering springs and the infamous RSR "modified" exhaust from Carnewal, which doesn't have TUV approval.
Must these modifications be checked by the TUV?
What happens if I dont do this?

Thanks.

[JMGS4]

Usually manufacturers of bolt-ons have an ABE (allgemeine Betriebserlaubnis) which you just carry with your papework. Anything else will have to be checked by the Tüv and entered onto your car paperwork. Louder exhausts is a frequent reason why cars are stopped in towns and if you've unregistered mods it could be very expensive depending on how "serious" it is! Usually it's bikes which have race-cans which get stopped, but a 911 with loud exhausts could well get the once over!