the inlet manifold is hugely restrictive on these cars, look at going down the gti 180 inlet and throttle bodie route, its not a straight forward conversion but has been done a fair few times with much improved results.
cat cams also do a fast road cam kit that really livens them up, also a 4-2-1 exhaust manifold will help get the gasses out quicker.
obviously all these mods would need a custom remap to make the most of things