Your first step is to learn that GDI and TDI are two different things.
On Diesel the injection point is also the ignition point, this is not the case on a GDI engine.
After you have understood that the fuel is not ignited before there is a spark, you can start doing something...

Changing the injection angle does not magically change torque on GDI or fuel consumption.
Especially in non-knock limited area it is almost irrelevant, as long as you don't spray way too early.
This is also why if you look at the actual "target" SOI maps they are almost flat with a single value, with a little change on higher RPM.
It simply does not matter all that much.

The limitation is "abbruchwinkel" or the latest possible EOI. This is the point where the pressure in the combustion chamber is getting close to the rail pressure, so no more meaningful amount of fuel can be injected/atomized. Obviously if you increase the rail pressure, this moment arrives later and because almost no one understands this, people tend to fit bigger injectors, when in many cases a bump in the rail pressure is enough to not only increase the maximum flow through the current injector, but also extend the injection window.
Limitation maps: KLWESABR/KFWESABRGM

For SOI the main limitation is the point at which the exhaust valve is closed. If you start spraying before that, you are spraying into the exhaust.
The second limitation is the volatile nature of gasoline. If you spray too early, the fuel gets deposited on the piston, it also gets heated a lot more and knock occurs.
Limitation map: KFWBHO1SMX.

So the tuning window is tiny. In fact, stopping spraying somewhere close to the "abbruchwinkel" is usually always a safe bet. You will not be able to ascertain whether you get a bit better atomization by spraying a bit earlier or later without some really serious equipment.
The ECU will actually by default take the requested SOI from the map, and if the EOI would be past the EOI limit, it advances the SOI until the SOI limit, after which if it is still not enough time to deliver the fuel, it cuts power.

So really, there is nothing that much to it. If you have bigger injectors, retard the SOI setpoint maps. If you have higher rail pressure, extend the "abbruchwinkel"...
If you think you're going to get anything else done without lab equipment, you're mistaken. The OEM is working on the last 1-2% because even 0.5% matters to them, you will never ever reach that level in the first place, nor does it matter.

It is actually a good idea to take the SOI setpoint angles from the engine you're lifting the injectors from.
The start and end limiters actually do not depend on the injectors at all. The first one almost only depends on the engine and the second one on the engine and the rail pressure.
The setpoint angles however mostly depend on the injectors, and less on the engine, especially if you calculate them as delta duration between "abbruchwinkel" rather than as an absolute value.