Right, I went for a Guarder spring (still about £6 cheaper than MadBull).
Guarder SP100 gives a FPS or 395-400 on High, 350-355 on Low, with Prometheus 6.03mm 509mm TBB (TBH installing the barrel didn't change any FPS readings).
(My chrono reads <5fps higher than ZeroOne in-shop one, and 20fps higher than GroundZero one)
I'm guessing mine shipped with wrong spring or had it's spring replaced, like yours.
I'm hoping what I read on forums about springs bedding in and loosing 5-10fps is true