[davidzo]

You can cut out everything that is more than 3-5mm away from the die. So don't make the pin-area bigger than that. you cant count + 2mm heatspread horizontally for the IHS (an A64 IHS is at least 2mm thick) remember always that the thermal dissipation of the die is more on the edges than in the corners, so the pin-area could be somewhat roundshaped.
4mm Pins are too tall, that won't bring any improvements especially on IHS CPUs, because the 3-5mm precept also apllies to the height. everything beyond that 4mm will have a relatively high thermal resistance and higher your milling costs, reduce the tubulence in the lowersector, ...
I would better make the channels a little wider ore more channels. If you wan't more flow don't change the length of the pins but the size of the channels, that won't remove the tubulence on the baseplate.
Pindesigns have a very high turbulence because of their restrictiveness. If you calculate the reynolds-formula the variable of the channel diameter is very important. the smaller the diameter of the channel the better the tubulences.
Turbulences affect directly the heat transfer efficiency.
Kepp that in mind when you try to make it somewhat bigger or less restrictive than the original principle (the nexxos XP). But it would be a great idea to remove all the restrictive channels on the outside...
To maximize channel surface and minimize thermal resistance the rest bp has to be as thin as it could be, but below 1.5mm the chance of bending is too high.

From my experiences 4mm is a good copper thickness, with 1.5mm bp and 2.5mm channels 0.35 sounds like a good pintopin distance