One source says 1300hp at 12.5lbs boost for take off.
The engine used a 9.75in impeller instead of the "normal" 10.25in impeller. It used an 8.58 supercharger gear ratio the same as the Merlin III, which used a 10.5in impeller.
The Merlin 32, which was supposed to be similar the Merlin 30 only with two piece blocks, was rated at 1620hp at 18lbs of boost. It's rated altitude may have been 1500ft.
The Merlin 30 was used in the Fulmar MK I and the Barracuda MK I, with the MK II Barracudas shifting to the Merlin 32. With only 660 of the Merlin 30s built perhaps they never got around to "approving" a higher boost?
I believe the 16lb boost rating for the Merlin III was for the Sea Hurricane?