so if they are 38mm that is better for low down torque?

How long is a piece string. Most EA-AU extractors follow a pretty standard Tri-Y design with minor differences with either 41mm or 38mm primaries and 51mm secondaries to a 64mm outlet, the Pacie PH4480's with 38mm primaries differ in that they are tunned lenght so a different pipe layout. 38mm primaries will help keep torque lower in the rev range while the 41mm will move the torque up in the rev range with better high rev flow. JMM do a race series with 38mm primaries and longer secondaries that extend all the way to the front pipe where the cat would normally bolt on, apparently good for low down torque and a broad power band.
Most people will tell you that extractors by them self will not give major gains but in combination with other mods more power gains are possible.
It all depends on what you mods you have and what you are using the car for.

He must take after you LOL you must be some car god if you can rebuild a car in 10 days, so before calling a moron look at yourself first.

If he wants to build a set of extractors at least that is achievable unlike a complete car rebuild in 10 days

End of rant