Going by the clocks and CUDA core count, a stock 780Ti should be ~12% faster than a reference Titan, but I would expect more scope for OC, boost and non-reference models turning up that are 10 to 15% faster than reference. So for here, some cards might be 20 to 30% faster than a reference Titan.

It will be the best single GPU card for here, and OC models might make it slightly faster than the combined cores of a GTX690. However, it's double precision performance is poor, 1/24 of SP (not good for WM). This, along with the 3GB GDDR5, actually makes it better for here - the 'up-to' 1/3 doubles of the Titan and the extra 3GB is of no benefit to GPUGrid. So the Ti comes without such unnecessary hardware overhead (bulk), making it more efficient and facilitating higher clocks (along with fabrication refinements and not hacking off an SMX or three). This also applies to gaming - it's from a mature process, is bigger and faster where it needs to be, and slimmed down elsewhere.

The 780Ti is more like the big brother of the 780 than the Titan, and should be ~27% faster than a reference model, mostly down to it's CUDA core count, 2880 vs 2304. Again, any FOC would be on top of that, so a 10% FOC card (if it turns up in the boost) would be ~40% faster than a reference GTX780.

In terms of performance/purchase cost and performance/running cost, the price is very good too, but this definitely depends on where you live. The purchase price is excellent in the US and many parts of Asia. Reasonable in some parts of the EU, but not so good in the UK - so no surprise there!

It's going to be a difficult card to follow...
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Most release date estimates range from Q2 to Q4 of 2014.
IMO it's far too early to give a Maxwell release date, and it doesn't really matter anyway!

Several things spring to mind about this:

I don't think NVidia will be in a rush to release.
The AMD competition has improved through revisions, tweaks and refinements, but hasn't swung the balance - it's still restrained by a 28nm lithography. Recent and potential price changes coupled with the existing GTX700 range, the new 780Ti (and possible future releases early next year) are enough for NVidia to keep/retain the status quo for many months.

Crunchers buy depending on what they want to crunch.
If you crunch OpenCL apps it's likely that your an AMD user and won't be changing to NVidia any time soon. While it might makes sense to get a R9 290X, you might not want to 'upgrade' to a GPU that offers less DP (MW).
If you are a CUDA cruncher, there is no choice but to stick with NVidia, so getting a GTX780Ti to even replace a GTX670/680/770/780 is viable, at least if you can pass on your old GPU.

I read an article about 20nm lithography suggesting TSMC may be able to ramp up production faster than with 28nm. However it's likely that they are going to have more customers and the die size of a handset or game system is a lot smaller than a high end GPU...

When a Maxwell GPU turns up, it's most likely to first appear as a Tesla and with a price tag similar to the last release - you won't be buying one to crunch on. In fact they might not even be available - pre orders to governments and data centers only?

Even when Maxwell's do arrive at your door, it could take some time for GPUGrid to get their hands on one, test it, develop and test the app for it.

So my gut feeling on this is that you won't be crunching on a Maxwell for at least 6month, probably 9months or more. Realistically about this time next year would be a good bet. That leaves the GTX780Ti looking like a very reasonable option and the rest of the line up at descent prices. It's actually a better time to buy than to wait.
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