Actually the quality control is the most important for the different levels of rubbers. The CNT are given the best QC because they require the best. Provincial players are professionals as well, but don't compete often enough in the World events to need that high of QC. Provincial when boosted a bit more than usual often plays and feels just like National only moderately boosted. Unless you're at the highest levels of play there's really very little difference from the heavily boosted provincial to regular boosting of national.
ESN has special QC rubbers for their professionals as well. Butterfly QC's their Tenergy for the CNT and players of Timo's caliber. I'm sure that all companies cater towards their fully sponsored players. ZJK uses 2.2 sponge for his T64. Timo uses 75 g on his BH and 71 g on his FH for T05. It's all about the best quality control which costs the manufacturers the most when producing. So instead of disposing of their products they instead will send them out for commercial uses. It's rare for two rubbers commercially bought to be exactly the same. There will always be some diference in quality, but it's just a matter of speed, spin, and control that most players look at rather than durability, sponge hardness, topsheet consistency (i.e. how often and where the topsheet is likely to wear down). Things like this all take into account the QC of a product.
As for judah's question - National is way expensive, but is well worth it. Provincial however is as has been said the best bang for your buck. The aspects of game are based on skill not on the rubber. While rubber can make a slight difference it is more often a technique error rather than a rubber's inability to do something well.