Why they've changed from solder to TIM ... could be higher heat in the chip, could be manufacturing costs, could be related to the smaller "3d" transistor technology they've been using which doesn't allow for solder to be used, could even be the solder not handing frequent temperature changes all that well and degrading over time. There's a lot of speculation on the subject, and I doubt Intel really cares because the processors work fine at their advertised speeds, giving no guarantees when it comes to overclocking. Obviously, the other point is that if they made chips that can easily overclock significantly they'd be eating into the sales of more expensive CPU's, so they likely would loose on providing the best possible heat transfer. The sale of K versus non-K CPU's alone is testament to their interest in that.

I'm not sure you can rely all that much on TDP figures ... there are no standards when it comes to establish these values, and TDP values between manufacturers vary. TDP between different models from a single manufacturer may certainly vary too. Plus the value is based on something like an "average load", not a stress test type of load which is where you see the throttling happening. Haswell is better at shutting parts of itself down when not in use, and that alone may take that TDP value down, it certainly takes the power consumption down. There are so many variables to consider here. But ultimately all that matters is not what the TDP value is rated at, but what temperatures you can get it at without throttling at a nice high clock.