Conclusion: Higher Speed, Higher Throughput
In renewing its TS-x59 series and specifically the TS-559 Pro, it was easy for Qnap to avoid mistakes. Take the quality casing and the interior of the previous series, add a mature and proven firmware, and exchange the processor for a newer model running 133 MHz faster. The move is of course evolutionary in nature, and it's a fairly logical step forward. From Qnap's perspective, simply adopting a new processor makes sense; it didn't incur any development costs, and the existing firmware is ambivalent to whether its commands are executed by an Atom D510 or an Atom D525 running slightly quicker.
The remaining question is precisely how much of a benefit is offered by the Intel Atom D525 dual-core processor. Considering the 133 MHz clock speed increase on the TS-559 Pro+, the boost to data transfer rates seems rather low. Quantifiably-higher transfer rates are mainly achieved in RAID 6 operation and where the transfer of data happens sequentially. But if a TS-559 Pro+ replaces a TS-559 Pro in a real-world environment, we'd bet that the difference wouldn't be noticed. Whether or not the processor makes itself evident depends on the application and user profiles; it's too small of a change to impress, though.
Currently, the TS-559 Pro and the faster TS-559 Pro+ are listed on Qnap's Web site. Both devices are also available from online vendors, and a closer look reveals a price difference of about $100 between the two.
Already priced up around $1000 without storage, these five-bay units are expensive. Near-term, users who've been toying with the idea of buying a TS-559 Pro will want to weigh the lower price with slightly lower performance in certain situations. Long-term, though, the TS-559 Pro will likely disappear (it was already deactivated on Newegg's site), only leaving the newer unit. It's 1.8 GHz processor doesn't offer any speed miracles on Windows networks, but a higher clock is always welcome, especially when it comes to the little Atom CPU.