hey,
my clutch costed like 660 sent to australia including a light weight fly wheel (all exedy),
intake - sounded heap sbetter maybe not any more power but a smother power band.
more expensive brands will fit better, look better, but in most cases will improve performance by very very little, its more about the rice factor.

as for your exhaust, i bought the same high flow cat, but haven't yet installed.

Xforce is a good reliable cheap brand, will give u improvements over stock, but wont give you as good gains as a jdm style brand.
overall though, power gains to money ratio is better, but not as ricey.
as per the twin loop i rekcon they are way to quiet, its like softer then stock. good for gains bad for the sound =P