Suspension
Lowering springs like will drop the ride height 1-2 inches and dramatically improve handling. The sping rate is progressive (gets harder as they are compressed more) which doesn't make the ride as harsh as super stiff springs. Progressive is best, Eibach makes a good product. If you just want to lower your car (stupid) then you can cut the stock springs, but without the progressive action, handling is not affected much. Some people don't like them though. The complaint is that they cause oversteer coming into a turn and understeer coming out (because they are too hard).
This brace dramatically improves cornering by eliminating flexing of the frame at the front end around the engine bay, keeping both strut towers aligned. Without the brace during hard cornering, the strut towers can flex inward/outward as much as a couple of inches. This is a highly recommended part if you want to improve handling. A strut-to-strut brace works great enough but for autocrossing demands, avoid the 3-point one that attaches to the firewall, although it triangulates the whole area and makes it all more rigid, you are risking cracking your windshield because the impact can put too much pressure on the wrong areas.
Subframe connectors make the body more rigid by connecting the front and back halves of the chassis.
T-tops especially sacrifice this rigidity and consequences are noticeable under hard driving conditions. Over time, constant flexing will cause problems such as squeaks and rattles, leaky t-tops, and improper door alignment. The connectors can be welded on or bolted on. Weld on's work better but cannot be removed. Make sure the cars weight is on the wheels when welding them on (at a place with using a car lift, not jacks). The reinforcing must take place when the body is in its natural position on the ground. This is a good mod, but only if you are becoming a more serious racer. Kenny Brown Double Diamonds are a must for autocrossers.
This prevents the axle from shifting left and right during hard cornering. With mods such as larger wheels, and stiffer springs/shocks, the stock u-shaped piece can bend and deflect causing rear end wobbling and possibly some tire rubbing on the fenderwell. The aftermarket tubular rod is much stronger than the stock cast rod and will make the car more stable during hard cornering and more predictable. Be sure to get one with zerk fittings so the joints can be lubricated.
These keep the axle from twisting under hard acceleration and prevent wheel hop thus improving traction. If money is a problem then you can improve the factory ones, just replace the rubber bushings with urethane ones and weld a piece of steel under the U-shaped piece to make a stronger box shape. This will be heavier than buying a set of aftermarket arms, but its better than stock and its cheap. A good mod if traction is a problem during hard launches. Again, as for all bolt-in replacements, zerk fittings allow for easier lubrication to prevent wear and noise. If you get these you should DEFINITELY SKIP the polyurethane bushings! The strong arms will not deflect, and the polyurethane will not deflect. This seems good but it will cause the wrong areas to take all the stress in hard cornering. Do some research and you will find that this combo can be damaging. Stay with the rubber bushings for this mod.
Increases rear axle stability for better handling, reduced axle hop and much better traction. Five pounds lighter than stock yet much stronger design. Go feel your stock torque arm and you will be surprised at how flimsy it is! This is expensive so save it for your last suspension mod, get the other stuff first.