Reducing IAS climb schedules were used in all the climb tests as per source documentation.
Here is the source data used for the Spit II climb. IAS is in actuality in MPH not KNOTS The line under knots is actually a misaligned strikeout line that should be going through the KNOTS label ... as it does on all other pages of the document. Given the test was only done to 16,000ft the IAS reduction was only in the order of 5 MPH and then only in the later portion of the test climb.
As reported previously based on timed climb segments the VSI on the Spitfire and Hurricane is over reading by around 500fpm.