Turbo comparison: 991.2 3.0 TT vs. 982S 2.5 VTG

B6 3.0 991.2 Carrera

Horsepower: 370
Torque: 332 lb-ft
Boost Pressure: 13.1 psi
Bore x Stroke: 91mm x 76.4mm
Redline: 7500 rpm
Compression Ratio: 10.0:1
Intercooler: Air to Air
Turbos (2): 49mm compressor/45mm turbine

B6 3.0 991.2 Carrera S

Horsepower: 420
Torque: 369 lb-ft
Boost Pressure: 16.0 psi
Bore x Stroke: 91mm x 76.4mm
Redline: 7500 rpm
Compression Ratio: 10.0:1
Intercooler: Air to Air
Turbos (2): 51mm compressor/45mm turbine

B6 3.0 991.2 Carrera GTS

Horsepower: 450
Torque: 405 lb-ft
Boost Pressure: 18.0 psi
Bore x Stroke: 91mm x 76.4mm
Redline: 7500 rpm
Compression Ratio: 10.0:1
Intercooler: Air to Air
Turbos (2): 55mm compressor/48mm turbine

B4 2.5 982 718 Boxster/Cayman S

Horsepower: 350
Torque: 309 lb-ft
Boost Pressure: 14.5 psi
Bore x Stroke: 102mm x 76.4mm
Redline: 7500 rpm
Compression Ratio: 9.5:1
Intercooler: Air to Water
Turbo (1): VTG 64mm compressor/55mm turbine

source (991.2) PorscheBoost - Comparing Porsche 991.2 Carrera, S, and GTS 3.0 9A2 engine output and turbocharger sizes
source (982) https://www.motoring.com.au/porsche-working-on-more-boxer-fours-101482/
I could not establish turbo size for the B4 2.0, if someone has that info, kindly fill-in.

Analysis*: All these engines have essentially the same internals, B4 basically being two-thirds of the B6 (longitudinally). Incremental power increases throughout the 3.0 Carrera line are achieved solely with larger turbos and tuning. By contrast, the bored-out B4 runs lower compression and has air-to-water intercooling; but its main difference is having one quite larger VTG turbo with more flow capacity than either of the B6's twin conventional turbos.

Now, that single VTG feeds all 2.5 litres of swept-area in the B4, versus 1.5 litres in each bank of the B6 for its smaller twin turbos. That seems like an advantage for the twin configuration, but I think a simple formula like turbo count*compressor-size/swept-area misses something about the physics of squeezing air molecules. Something that, crucially, can make one big turbo more powerful than two smaller ones. Because at peak power the big turbo's larger internal volume means air molecules are squeezed down less for the same flow; equals less heat generated in the compressed stream flowing out of the turbo. Therefore a cooler, denser charge for a given level of boost; more oxygen/fuel drawn into the cylinders on intake; more power. Of course you need variable vane geometry allowing the larger VTG to spool-up like a smaller turbo for reduced lag -- that, not ultimate power, was the historical advantage and motivation driving twin turbo set-ups pre-VTG (along with packaging and cooling considerations).

I think this single-but-large-VTG turbo effect helps explain the 718 S ability to 'run above' hp rating and power/weight calculations.

*this is my best -- but entirely non-engineer-reviewed -- analysis based on reading everything I can find online (which is not all that much!) on VTG and engineering aspects of twin-turbos versus one larger VTG (or twin-scroll) turbo. If you have corrections or addtl. insights, please contribute to this thread.