You mentioned the z tune stuff. Always thought the Z tunes were in excess of 200k when all was said and done but could be wrong.
I'd challenge you to find somebody who is making 400awkw who has spent less than 30k while building the car. Most people end up spending close to the original purchase price of the car especially when you start to through in bigger brakes, suspension, driveline etc. Making 400awkw is easy yes but making it reliable. That's why people spend the extra money.
Making a bit over 350awkw and making 425awkw like that GTR is worlds apart.
Amen 1 injector upgrade for all your future needs.