Hilton recommends the grey especially when the car is tuned. Contrast strongly recommends to use the OEM even if the car is tuned.
Both are respected, top rated and very knowledgeable tuners.
I'm sure each has their own reasons why and what they're recommending.
Yes, the OEM seems to have quicker response on partial throttle, while 'Gandalf' feels stronger and balanced across the rev range.