Other than the usual (and some unusual) longevity enhancements, and some very careful assembly, the following modifications made a difference to my engine:
3.4 pistons/cylinders, actual CR is 7.3:1
GT2 Evo cams, timed to factory spec
Very massaged heads: 52 int. valves, 43.5 ex, expertly ported
Extrude honed Carrera manifolds with 70 mm TB
Mototron 60 lb/hr injectors running at about 3.2 bar
Holcombe headers - 1.75 in, with Tial WG and Borla muffler
Garrett T-66 turbo, by Kevin
DTA ECU, tuned by Dave Capurro (while I "helped")
The engine has three maps: 92 octane pump fuel at 0.9 bar, 103 unleaded at 0.9 bar, and 103 unleaded at 1.2 bar. For the high boost map, there was a flat torque plateau at about 618 ft-lb from 4500 to 5250. Peak power was 622 hp at 5500. Boost apparently comes on hard from 3750 to 4000; the torque rises from 400 to 552 across that span. Boost is controlled electronically, through a solenoid controlled by the ECU, with a 10-15% reduction in boost at part throttle -- something I wanted for my road racing (track day) application.
The middle map (0.9 bar with 103 unleaded), which I'll use most often, shows yet a flatter torque plateau of about 500 ft-lb from 4500 to 6000, and a power peak of 550 hp at 6000 rpm. The pump fuel map shows numbers that are slightly lower, due to further timing retardation.
There's probably a lot of "show-off" power left on the table with this engine. It could easily have run a lot more boost and I'm sure would have produced horsepower numbers well over 700. Also, the cams caused the torque to peak earlier and fade more than I expected above 5500, hence the horsepower number could be significantly higher if the cams were designed to produce torque higher in the rev band. But I'm happy with it as is and it should be an incredible track performer with very large and usable torque range as well as a conservative margin of thermal safety and AFRs in the low 12s across that range.