You are picking what suit better to your thesis and once again put a layer of complicated arguments to dissimulate this fact.
You are right there is the intake ram effect. Just like exhaust gazes propulsive effect ... and a lot of drag with the cooling system that did not take into account the boundary layer drag even at a late stage in the war.
It does not mean that you can add all the Power gains RR refer in its doc and then say that the merlin had the a total of BHP equal to the cumulative effect measured on the test bed.
Note that the intake ram effect went as a benefit only after a major redesign witch hve to be dated
The diag you show is difficult to interpret as it it shown put out of any contest. And we are talking abt early war tech not late achieved Merlin boost. In 1944/45 the war for the Spit has switch from high alt air interception to low level mud fighting and interdiction witch favor over-boosting. (we also know that those level of boost proved unreliable and were lowered on the field - the griff engine being put forward has the 2K HP piston eng - Oh... and yes there is the Merlin Hornet but... wait is that not an evidence that DH has superior engineering capacities in term of aero when Supermarine despite strong gov support only produced contestable design ?)
In term of FM and drag for the spit, just catch the six of a 109 in the game and follow him in her maneuver for a min. If you are not laughing after 30 sec and still need to be convinced, pls take a similar ride in the Excellent Hurri we hve for now.
So no drag reduction! My humble opinion wld bend me in the direction of some change toward the set of modeling eq that might not apply to a 300mph fighters (in fact modelizing the Spit wing as a trapezoidal wing of similar wing surface/wing root chord and thickness wld lead to better accurate results without changing the overall FM engine). But this is pure guess as I don't know anything abt CoD FM engine
So, to resume my self : you can't take a pce of this or that and build a convenient result
For example if you build your opinion on the spit only reading my posts here you'll conclude that the spit was an antique machine unsuitable for fighting even if this is not what I am thinking.
I repeat myself : The spit/Merlin were great design but with some major flaw that a simple look at history puts in perspective (search the web for a Elliptical winged B2 or floating carburetor PT6 turboprop or a Malcom Hooded BlackBird
)