I see the S.P.O.O.S.E.S.* is back in full session.
Please note that what ever boost level you can get out of a Merlin 45 or XX series engine is for about 5 minutes only. The 30 minute climb ratings and the max cruise ratings didn't change.
Merlin 30 was good for 1260hp at 8750ft. the only way you are going to get a Fulmar to 310mph at 10,000ft is with a large rocket strapped to it.
* Silk Purse Out Of Sow's Ear Society.
The Merlin 30 was rated at 1360hp at 6000ft @ 3000rpm with 12lb boost and FTH with ram was 6800ft = 268 mph TAS in the single test result we have access to. . The Merlin 45 was rated at 1515hp at 11000ft @ 3000rpm with 16lb boost. It follows from this, that a Fulmar with a Merlin 45 producing 1360hp at ~12000ft (with ram) = a TAS of ~290 mph but at 16lb boost it would be developing 155 extra HP at 12000 which should provide another ~20 mph for ~310mph TAS. The Merlin 45's climb rating was at 9lb boost but it could maintain 9lb boost for another ~6000ft higher altitude which would more than compensate for the slightly reduced climb rate at lower altitude.
The data for a Merlin XX would be similar.