Header tanks, or de gas tanks don't leak due to heat, they respond to pressure only.
The turbo cooler is a gimmick imo, some report it has made a big difference, I think this may be more psychological than anything when you consider the temp of the water exiting the turbo and surface area of turbo rad compared with main rad.
First get a decent cap, make sure that you fill only to the low level, are you using a alloy or copper rad ?, if not get one asap, std rad is going to struggle.
When I made a pipe convertor to go in the turbo return on my 2wd into a 4wd tank, as you know the pipe is much larger on the 4wd tank for return, I made a space for a 1/8 NPT sender to be screwed in so you can monitor turbo temperature, so you can see exactly what is what.
tabetha