Broadwell-K chips are basically the only desktop Broadwell chips coming out. You'll get the performance boost from architectural improvements, which can vary but typically are around 5-15% depending on the app, if we extrapolate from past data. I would speculate that Intel wouldn't downgrade the stock specs for those Broadwell-K chips from their Devil's Canyon predecessors.
On the other hand, Intel will be releasing locked Skylake chips at the same time, but no one knows what the pricing for components such as DDR4 will be at the time of release, but rumors are it will initially come at a premium.
I don't think the 2500K is so long in the tooth to warrant an upgrade right now, but the 6900 series of Radeon cards is either in that territory or approaching it. They are approximately equal to a
Radeon 7850. It is severely outclassed by even $300 GPUs like the
R9 280X(aka 7970) or
GTX 770.
Mini-ITX boards have the same exact socket as ATX boards. The matter is finding an older LGA 1155 mini-ITX board