To be honest I can't see what all the fuss is about.
Determining line diameter via engine size is (IMO) an archaic method devised back in the days when there was a comparatively limited numbers of standard capacities and a fairly narrow power output across each displacement (Just talking aerobatic engines here, not TR, speed etc)... I always assumed the line tests were not really related to the expected pull during a normal pattern but more designed to cater a loss of line tension and the model flying free flight across the circle and hitting the lines on the other side. To me a 4 pound model doing 60 mph across the circle, hitting the lines with a merco 29, LA46, AXI/Scorpion whatever is going to exert the same pull on the control system, the only common element here is the weight. (A Ton of feathers verses a Ton of Lead). With the now common use of mufflers the overall weight of a wet power plant has increased (Along with model weight by the same amount). Due to modern engineering outputs have gone up at the same time. Pull tests based on model weight is the only sensible solution. I don't fly competitively yet but to me the main issue here seems to be the extra work involved for the organisers. Just going by engine size was a lot less work than having to weigh the model and apply a bespoke pull test to each one. I may be missing something here but if so I can't think what.
TTFN
John.