Ok, I would go the t517z with the 10cm exhaust housing. The t518z is a better turbo but your'e not asking that question. The HKS GTSS is good but is much better suited on an RB26 twin turbo upgrade. You are not going to see much of bigger power increase over the T28BB.
Tuned properly you should see about 350hp with the t517z on about 16psi In the Garrett it's very similar to the 2860r. It is worth remembering though with the t517z there is no water cooling or ball bearings, so long term reliability could be a consideration. Their wastegates open the other way, this makes using split dumps almost imposible.