Late to the conversation... but I'll add to this. We have a warehouse in China, where we have our products (toys) tested and checked before being shipped out. Once they leave China, there is really no recourse... and it saves us thousands to do it this way. Being able to return faulty stock to the factory (they don't like that) ensures that we get better stock in the first place.
When we first started out, the average worker was earning about $0.70 per hour. We told our agent to double it at least, but she warned us against it, saying that if we raise the price, a couple of things would happen. Firstly, the workers would be suspicious that because we are offering more, that we wouldn't pay them (too good to be true)... so they wouldn't show up for work. Secondly, if they did show for work, we would obviously pay them. However, word would spread in the local village, and the workers would be attacked by their neighbours for earning more. The insistence on equity where we frequent is almost religious. The workers were fine with the going rate, so we had to toe the line.
So we paid cash bonuses instead, and they were ok with it...
China is a very opaque place. My experience is quite narrow, but without an outside observer any additional amounts paid would go straight into the factory owner's coffers. Unless you have a contract in place, and an agent overseeing better conditions, you will get assurances, but nothing more. You will be told how much better conditions are, but nothing will have changed, because there is no incentive to.
Again, I think it goes back to my first answer. The Chinese would resist companies like Fairtrade looking over their shoulder. Especially as places like Yaqi are privately held. Things might be changing, and we do get SGS testing on some off our products (for lead paint), but we have to organise that ourselves...