The TT comes with fairly nimble suspension from the factory, however most enthusiasts will agree that there is much room for improvement. Safety and practicality concerns lean the TT's suspension towards riding high and understeering in turns.
Fortunately, there is a broad aftermarket for suspension on the TT. I'll convey my knowledge through a series of notes broken down into specific suspension related topics.
Choosing a suspension setup:
-The only difference between coilovers and a spring and strut setup is that coilovers offer adjustable ride height. THAT'S IT. Some coilovers have dampening adjustability but so do soem strut/spring kits....adjustability is usually the only reason they are more expensive and more commonly used. Only real benefit to coilovers performance wise is the ability to corner balance.
-Running lowering springs on stock struts will most likely wear the struts out faster and make for a bouncy ride...I don't recommend it.
-When buying coilovers/springs and struts, replace your strut bushings and bearings as well while you're at it. They're not expensive, but most likely could use replacing.
-There are many good spring and strut manufacturers including big names like H&R, Koni and Bilstein. Cheap coilovers include brands like Vmaxx.
-Some Golf/Jetta components may fit the TT, however the two ARE NOT the same.
-Do your research as to what coilovers/springs go how low. There are many brands and they do not all produce the same amount of lowering. There's a post on here about a coilover pic thread on Vortex that I started.
-Lowering will increase your rear camber somewhat. This in itself will not severely affect your tirewear. Unfortunately, on Quattro TT's, the wheels "toe in" (point towards each other) when the camber increases which WILL destroy tires ridiculously fast. This will require a set of Kmac bushings ($200-$300 on TTstuff.com) to correct. You can also use adjustable rear control arms to do the same thing. FWD TT's do not need to correct the camber as their rear axle is different and does not produce the same "toe in" effects.
-Lowering can put your engine components at risk from bottoming out. DieselGeek sells a metal skidplate to give your oil pan and other components some protection ($300).
-Many coilover kits come with adjustable rear spring perches, but often to get desired lowness the rear suspension must be run without using these perches. You can run them this way, but it does not allow adjustment of the rear ride height.
Handling improvement; reducing understeer:
-The single most effective way to reduce understeer is a rear sway bar. These come in many diameters, but generally it is advisable to go with a 19mm rear swaybar if keeping the front swaybar stock. Larger sizes are primarily used to complement aftermarket front swaybars.
-Control arm bushings are another way to help expunge understeer. Some people replace the front lower control arm bushings with poly bushings to firm up the steering, however poly bushings are said to wear out quickly in these locations for some people. Another option is a set of DEFCONS from MCPi, which adapt the control arms to a previous, more desirable design (read the site for more info).
These are all the mental notes I have about suspension. If you have anything to add, feel free to contact me.