In the case of the 100-400 no one really knows if there was a 'silent fix' or not but it seems that there is a lot more praise/a lot less complaints of the lens now than there was pre 2007. It wasn't uncommon to hear about the lens being soft but having owned one (assembled in 2007) I don't see softness at all, and in fact many others don't as well. In the end there is no way to really give you a deffinite answer, but it does seems like the lens got better. Best advice I can give you is to buy the lens under the condition that you may return it for a full refund upon testing it for yourself. If it's a used lens, be sure to get sample images prior to buying as well.