THe blitz kit is a good item, however the step at the end to suit the stock piping can promote a bottle neck into the pipe that goes to the throttle body. Standard piping size is 2.5" and the kit looks to be 3". I'm sure it's not a big deal but something to be mindful of.
For that reason, along with mandrel formed smooth pipework, I preffer the JustJap Cooling pro 'stealth' kit. The end tank design is a little more clunky but realistically not many will be pushing over the 220kw atw mark anyway. The stock GTT side mounts are capable of flowing close to that anyways, so it then becomes a topic of astethics.
It's basically a copy of the blitz kit but in my opinion looks like a nicer product. Exposed welds to be look unfinished.
P.S. it doesn't come with cooling pro on the core either.