So, there is not much clear answer lying around the internet, but I am still seeking for one. Supposed if I were to compare, a Desktop with Intel i5 6700K vs a Laptop with Intel i5 6700U; with the desktop being underclocked & TDP limited down to meet 6700U being overclocked.
In general sense, will both Desktop and Laptop perform the same or will there still be performance difference?
There is no such CPU is the I5-6700k or I5-6700u according to the Intel Ark which would be fairly authoritative. In fact, I could not find ANY i5 or i7 6th gen CPU's with the same model number but in mobile and overclock-able versions.
Were such a beast to exist, it would most likely perform identically if according to the Intel Ark everything about the CPUs were the identical other then the clock speed - although that is not a given. Looking at the i5-6300U and i5-6300HQ - both mobile processors, they are entirely different beasts although they have similar names/numbers. On the other hand a comparison between the i5-6500 and i5-6400T show them to be pretty identical save for clock speed, TDP and thermal disipation requirements.
This is unsurprising as Intel test their CPU's and put them in different bins depending on how they perform - ie the k and non-k version of the CPU likely come from from the same line with small variations in the quality of the materials being responsible for different performance.
I did ask a tangentially related question some time ago about about power consumption of a couple of similar CPU's who's answers which might be of interest to you as well.
