Correct, It will help top end power. Low end spool up will be slightly inhibited as you will lose some of the pulsing of the twin volute.
Reducing back pressure is important. 1.2psi back pressure or lower to 1.0psi Boost otherwise you are leaving HPon the table.
Running a waste gate or getting a larger turbine wheel or housing will reduce back pressure. You add more exhaust gasses/ from fuel/ boost/ higher RPM and it gets constricted.
The stock hx35 is about 58mm exducer on the turbine (25.4mm = 1") =2.3" figure you have a 7mm shaft, + hub (lets say 2x 3mm=6mm) and a number of blades on the shaft. So with bad math we get to about 45mm with blades in the way. So thats about 1.8" with blades. Enter the wastegated housing... 17mm to 22mm are common sizes, S300 turbos usually have 2 of them.
So with the wrong size turbine/wastegate/ turbine housing its common to have over boost (less efficient), over speed of the compressor wheel (reduced life from centripetal forces on the wheel = deformation, oiling problems, less efficient, more wear), and restriction to the flow of the exhaust). Its about flow in and out of the engine (boost does help). Think of a water hose. Low pressure... high flow. Kink the hose and you get 40-60psi.... and no flow. There is a sweet spot and a good place to start is 1.2back pressure to 1.0psi boost.
No one would recommend ditching a 3" or 4" exhaust and putting a 2" exhaust on just to overspeed the turbo, make it less efficient and wear out your turbo faster to create greater pumping losses in HP and a pressureized engine/box). But that is what blosking off the wastegate can do for you if you do not know your boost /drive pressure ratio.
Who has measured their back pressure/ drive pressure and lowered their boost to bring back pressure to a more beneficial #? Please post your experience.
Looks like you did a nice job. The more you pull out of that center divider the more the sides can equalize pressure and gate from both sides, but the more it disrupts the pulsing and therefore the spooling of the turbine. If more power is your goal do aggressive machining as pictured. If you want a small reduction in drive pressure/ with less reduction in spool go milder.
Again the machine work looks great.