I see 750ti price has jumped £6-7 since this morning so to quote Shang-Tsung "It has begun!" lol
Steal the crown is not right yet but it has levelled the field for now (with high end Maxwell having a chance to steal the crown).
You see mining isn't about KH/s per slot/per computer it's about efficiency, unless you only plan on having one rig (which a number of people do but they're not exactly real miners, there like "gamers" with APUs), hence why people currently build R270 rig farms over R280/290 rig farms, a simple way to look at it is like this:
Except you get into the general problems with logistics as above, anyone that has space for 2 rigs, has space for 2 rigs, meaning 2 x 280x based computers will yield more profit and more significantly it will yield more hashpower in important times like at launches of new coins. If you have space for 5 rigs, 5 x 280 rigs will do a lot more.
Ultimately the investment cost is next to irrelevant, any given "hash" will pay itself back at the same rate, one computer or 10 pay back in the same time, you just make 10 times more profit from the day after they pay themselves back.
Who would chose having a room or warehouse or anything full of 3 times as many setups, which also needs 3 times the amount of work, building, monitoring, testing, switching, etc, etc. It's more work for no reason at all.
No serious miner would build more slower rigs rather than less faster rigs, there is zero point in doing so.
As for efficiency, my 7970 doesn't use 250W and undervolts while overclocking significantly. Almost everyone who uses a 7970/280x is both overclocking AND undervolting to get less than 250W power usage. Likewise your numbers seem to bump up the 750ti from 250 to 300, a 20% bump over what I've seen stated.
As for the high end Maxwell, NO higher end card will be more efficient than a low end one. In reality at the lowest end you can simply choose the best balance for power usage, you pick a memory bandwidth and scale shader cluster numbers to perfectly use it. At the high end, say with a 7970, you need more than a 256bit bus so you spend power on a 384bit bus but don't saturate it. That will never be as efficient but with a 256bit bus it would be woefully unbalanced. High end you have to deal with max die size vs yields vs power and compromise low end you don't have any hard limit so you just make the most effective card in that bracket possible.
The likely hood is that if the newest to date card, that don't forget was launched in Jan 2012 only JUST, and I really mean JUST manages to match the 7970 for mining efficiency..... where do you think AMD's next gen might be. In fact, given their mining capability it could be an area they've been thinking about architecturally in that past two years for all we know. The next gen might spank the current gen silly. Nvidia's almost certainly most power efficient card on their next architecture is only on par with AMD's two year old architecture, that paints a picture of an extremely low likely hood that they'll match AMD's next gen architecture.
