When we developed and tested the TA headers years ago there were many considerations in the design that we found that were significant, and one was the size of the tubes and especially the opening on the 3 bolt turbo flange.
The turbo opening was enlarged from the size in a stock header, but it was found it hurt exhaust velocity so we went with the stock size opening to maintain good turbo response.
The Poston header, and the Headman/GSCA headers, use a larger diameter collector pipe on the passenger side header which also adversely affects flow velocity, not really bad, but noticeable on some builds.
Depending on the turbo, if there is a lag in response we found installing a smaller orifice, or an insert, in the 3 bolt opening would help.
Not really pertinent to the subject headers, we did fabricate a few sets of custom race headers for high HP/RPM applications, and we found using a 1-5/8" primary tube instead of the 1-1/2" stock size tube at the exhaust flange provided significant gains with big exhaust valves and race ported heads.
With all that said Rob, my opinion is your build and turbo will be fine with those headers, especially when you step up to alum heads with way more exhaust flow!