The standard MK2 manifold on Aussie cars is a joke, you could block the gas flow with a 20c piece. It looks like a decent size pipe but it tapers down to a pollution legal (strangle) size.
https://www.vwwatercooled.com.au/for...405_124745-jpg
Here's the dual pipes in my MK1.
Attachment 48487
The dual downpipe manifold from a MK3 is a great improvement. It depends on what mods you've done to the motor as to how much extra power you get but 5hp on a standard motor sounds right.