Josh, your thoughts of earler are in the right direction. When it comes to DI with SC we want as much window as possible to inject fuel (as this is normally our limitation to power) and we want as much time for that fuel to try and reach a homogenous condition with the air before it burns.
Remember - SOI is not when the injector opens - it's when we "ask" it to open. Hence, it should get earlier with higher RPM, not because it's actually happening earlier, but due to mechanical limits of the injectors that we need to account for.
All of my testing was done with a stock LT1 camshaft back in 2015, but since PD camshafts have similar overlaps, I wouldn't expect any differences. We ran dyno pulls increasing SOI at 5 degree incements on the engine dyno (not chassis) from stock settings until power flatlined, which happened with SOI at about 360-365 at 6500 rpm. (lower at less RPM of course) Also note that changing SOI does not change your fuel flow, only a change to PW can make that happen. While there are settings for EOI they aren't a hard limit, stretching your PW to longer than your allowed injection window will inject fuel outside this window.
Below are my "safe" settings for PW while staying withing the window (also based off rpm)
Engine RPM MAX PW (ms)
2000 20.0
2500 16.0
3000 13.3
3500 11.4
4000 10.0
4500 8.9
5000 8.0
5500 7.3
6000 6.7
6500 6.2
7000 5.7
7500 5.3
8000 5.0
8500 4.7
9000 4.4
Preferably I like to stay within those ranges - but on DI only I'll admit to pushing a bit past this in the quest for power.
So, in short - if you're pushing the limits, I've always found increase in power when SOI is early and correct for that RPM, which in part lies the solution and the problem (finding that exact number).
Since we now run port and full E85 on our cars, importance of these DI SOI settings drops a bunch and I leave most of the DI fuel settings as stock. We keep enough PW on the DI to maintain all the knock benefits that it offers, then provide a nice homogenous port fuel charge to make as much power as possible.
Hope that helps!