A lot of this is just demagoguery. If you give the test to GC holders who are not close to N400 stage, they will fail in similar numbers. They pass because they prepare. Do the same to natural borns ... give them time to prepare, give them motivation to prepare and they will do much much better.
Comparison with other countries ... didn't we already know the mental math skills of our population from the housing crisis that we need a formal test for them?
GC: EB1: 1999 ==> 2004. 3 yr delay due to lost file. N400: San Jose. Apr'09 ==> Aug'09.
I am not a lawyer, forum police, moderator, moralizer or an immigration officer. If I sound like one, use "your" best judgement.