Janspeed manifold fitted - surprising result

[dave23572]

Fairly bog standard ZS180, only performance mod is the ITG induction kit.
After ages of deliberation I finally bit bullet and got myself some Janspeed manifolds as the first of a planned list of mods. The idea being to do each mod one at a time to see which ones make most difference.

I also modified them by spending half a day with a Dremel to fettle down the lumpy interior welds to hopefully make it flow better.

So ITG done (a year ago) and now the manifolds have been fitted on their own. Cat-back to come later.
I was really looking forward to the results.

And the results?
I literally cannot tell the difference!
What a huge surprise and letdown after all that money and effort.

What could be the reason for this?
I've seen before/after DYNO graphs showing 10BHP or so with this mod.