even if you manage to install your freezer 13 it won't cool those little mosfets.
The HP heatsink isn't that expensive, at least not when you get one used off ebay.
It has a Delta AFB0912VH 92x25.2 mm 3200 rpm fan and a very special feature which you find on several HP heatsinks:
air is partially directed to the mosfets which sit on the left and top side around the processor socket.
These will get hot under load because they have to provide a serious amount of current to the CPU, the Xeon itself isn't that critical.
If this project is just a kind of research or "let's play and see what happens" kind of experiment, then go on with whatever type of cooler you want, but if you want to have a reliable workstation build I recommend to get the performance heatsink.
The mod you refer to shows us how to perfectly ruin the airflow in a well engineered workstation.
And yes, the fifth pin is an additional ground connection which tells the system board "hey buddy I'm performance heatsink, let's rock...).