i personally think the laguna one is strangling it and the pipework isnt doing much for it either, lots of bends and different sized pipes
my pipework is 2.5" from turbo to throttlebody, all mandrel bent alloy with minimal bends, an exhaust place is probably your best bet for that, or i can speak to my guy and get a price in stainless for you? (no money for me admins btw, just helping him out, he can pay him and pick it up, or il go with him)
Core wise similar to mine (or even bigger) would be good, as long as it doesnt cause lag or strangle the flow or fail to cool your onto a winner (ebay ones do actually cool very well just made cheaply)
just remember when thinking about all this, main things are you want is to flow well and you want it to cool, the cooler the better ovbiously
EDIT: remember, bigger isnt always better