I've had my SP3 for just over 4 weeks now. Before ordering, I was concerned about the heat, fan, throttling issues previously reported. I know someone who works in Quality on the SP3 at a factory. He said minor hardware changes are a common thing - the most recent being a change to the fan design and type of paste used to bond the fan to the CPU. The last changes he knew were done in September.
In this respect, it's handy to know the 'birth date' of a device. I guess some retailers, might have had certain models on their shelves for sometime. This makes a case for ordering direct from Microsoft, or a retailer with high turnover.