Whoopsy:
Haha, it doesn't quite scale that way, not even close to linear. Remember the turbo is only one part of the equation, piping, cooling and compression ratio also plays big roles.
MP4-12C has a 3.8l engine, lets say a nominal figure of 100hp per litre, something very doable without much trouble, so 380hp as a NA engine? slap on a pair of turbos and an extra 240hp. THe P1 puts out about 727hp, so another 100hp out of the engine. Still very doable if they were only lightly stress in the 12C.
The turbos in the McLaren are not big, quite small actually, lets say they are pushing about 300hp max each. In the 12C configuration each will only be asked to supply 120hp, not quite 20%, more like 40%. In the P1 configuration they will still be pushing only 150hp each, 50%. Right around the ballpark I speculated on the post above.
You are right about there being more to it than that, but trust me I know what I'm talking about, one of my GTRs (an R34) ran with 5 different turbo configurations, including twin and singles and I spent plentynof time on the dynos and looking over compressor maps etc.
You are incorrect about your assumptions regarding how power is added. All the air that goes into the engine goes through the turbos, so if the engines are 600bhp, then they are flowing 300bhp through each turbo. Note that turbos are usually measured in airflow in lbs/min not bhp, since all the turbo does is flow air whereas bhp is a more complex product of engine design, BMEP/efficiency, advance and fuel quality. Volumetric efficiency of a turbocharger is also very poor when used near maximum etc.
The bigger the turbo the more efficient is at passing greater volumes of air, but the downside is lag, both in terms of transient lag (ie speed to react), but also boost threshold ie what rpm it comes on boost. As an example, I had the same 2.6l RB26 engine in two different GTRs and with small stock turbos is came on boost fast and low in the range <2500rpm, but could never make more than about 460bhp (from memory), then with bigger Trust T617z, it spooled up about 1000rpm later, but made 680bhp at max boost. When a similar engine was fitted with Garret 3037s, it made as much as 1015bhp, but only came on boost at 5500rpm (it was built for drag-racing).
McLaren like every car manufacturer will have chosen a turbo size that runs about 60-80% of capacity in normal use, but that is small enough to spool up quickly. This leaves spare capacity for upgrades. No one ever fits turbos that run only a small amount of their designed volume as they would be far too big and result in terrible ability to respond, both transient and rpm-based.
(NB When describing lag I mean both types, both at what rpm the turbos start to produce meaningful boost when at low rpm in a high gear and transient response, which I mean as when you are already at high-rpm but the turbos are not spooling and suddenly are required to spool. The bigger the turbo in relation to engine size the worse they are at both, hence why VTGs were invented to try to partially get around this issue.)