The energy-efficient Haswell-based processors prove the incredible flexibility of that microarchitecture which allows developing solutions with dramatically different performance and power consumption properties. It turns out that technically identical processors, which have the same design of the semiconductor die, can be marketed as high-performance or energy-efficient products depending on their clock rate settings, particularly the turbo mode settings. The Haswell’s integrated voltage regulator was originally designed to be versatile so it can supply as much voltage as needed in specific conditions.
Considering that, we can't really expect the low-TDP processors to have some special properties. They do consume less power and generate less heat in comparison with comparably priced ordinary models but can hardly be more economical than the cheaper processors with standard TDP. Talking about the tested processors, it means the Core i5-4670S is in between the Core i5-4590 and Core i5-4460 in its power consumption and heat dissipation while the Core i5-4670T is somewhat less economical than the Core i5-4460 or Core i3-4360.
All of this is directly linked to the performance of the T and S series processors. Their energy efficiency is achieved by simply lowering their clock rate, so it is no wonder that the Core i5-4670S is comparable to the junior standard-TDP quad-core processors while the Core i5-4670T is just one of the slowest quad-core desktop Haswell-based products. In other words, the quad-core Haswell-based processors with a TDP of 65 and 45 watts do not have a better performance-per-watt ratio. They are the same as their standard-TDP counterparts in this respect. They also have a standard integrated graphics core of the GT2 class.
It turns out that the S and T series aren’t much different from ordinary LGA1150 desktop Haswell processors and we should judge them by their clock rates rather than model numbers. Their prices will look too high then, especially the price of the S series which differs from the regular series but very slightly. The more economical T series may be more interesting because it has no analogs in the regular series.
So our recommendations about choosing CPUs for high-performance but economical computers of compact form-factors go like this. Quad-core Haswell processors with clock rates up to 3.4 GHz will fit into the 65W limit. In this case, the optimal price/performance choice is the Core i5-4570 or Core i5-4460 rather than the Core i5-4670S we've tested today. But if you need a quad-core processor with a TDP of 45 watts, you have to consider the T series because any standard Core i5 consumes more. Of course, you can also take an inexpensive Core i5-4430 and downlock it using your mainboard's BIOS settings but the Turbo Boost technology gets disabled in this case. The T series, on the contrary, has an aggressively set-up Turbo Boost which can provide significant performance benefits in low-load applications.