that depends on the actual manifold... if it flows well, then you wont gain much, but if it sucks, then you could still gain a bit from a header... either way I would suggest you save up for a turbo, and go that route.
but as for the rest of the exhaust I believe its different, ours doesn't jog over the the left after clearing the oil pan like that. however, your mid pipe thing sort of brings it back over the the right, so I dont know... a US market "cat back" might still fit on there for ya with modification