I'd say treadstone is lower grade than garrett. They're definitely above all those ebay brands you see like the FMS. I was trying to balance price and internal fin design, and treadstone seems to have a pretty good product in that they have a relatively dense internal fin setup.... Good for cooling, with about 2 psi drop across the core.
The only other bare cores I could find were bell and those were super expensive like the garrett's.... North of $400 just for the core, plus end tanks. My treadstone core was $268 shipped.
All in I'm going to have about $600 into the FMIC kit (core, pipes, v-band flanges, brazing stuff, couplers/flanges), and another $260 into the bpv, plus my time. The core is rated to support 475-500 hp, so I think I'll be keeping the turbo in the 50-52 lb/min range. If I do go bigger it will just be making the same power with lower boost.