I agree that 70 mm is as big as a TB for the 2V needs to be, however it has nothing to do with the MAF's inability to measure the airflow.
Any engine running n/a can only ingest as much air as its displacement and volumetric efficiency allow. At an unattainable (running n/a) 100% VE and 6500 rpm that would be 528 cfm, or 2375 lb/h at STP¹--comfortably below the stock MAFs 2650 lb/m maximum. At a more realistic, yet still high for the 2V, 85% VE the most it could digest would be 2020 lb/h, well below the MAF's capacity.
The problem is, and why larger than 70 mm TBs can hurt performance, is to be found in the charge velocity; the speed of the air flowing into the plenum. At a point as the TB gets larger, when running at WOT, the charge velocity will fall to a level at which air flow into the cylinder is reduced--this causes reduced power at WOT and higher RPMs. Also reduced is the effect one intake pulse's mass has on pulling along the one behind it.
Chapter 7.6 of John Heywood's Internal Combustion Fundamentals explains this in great detail. It has nothing to do with the MAF's capacity.
¹ - STP = Standard Temperature and Pressure. 0.0°C and 1 atmosphere, 32.0°F and 14.7 psia.
- 2003 GT, UPR X, FRPP 24lb/h, Magnaflow, PP 70mm TB & plenum, Delta Force tuned,
Steeda UDPs, Ralco flywheel, RAM HDX clutch, 3.73s, 262 rwHP/305 lb-ft.
New ride (7/1/2013) 1998 Mercedes SL500-5.0L 32V VVT 326/347 HP/tq