Well - it's out of warranty, so everything is in your hands now.
The dealership we got it from has been real cool and didn't think the engine threw the rod due to the longer OCIs.
I can't find any lack of logic here.
They said the original engine did have some sludge in it but since then they installed a centrigual oil bypass as a recall and not much soot shows up in the UOA.
You were changing oil at the recommended OCI (400 hours) per the manual. If it sludged up, that would be a design issue, would it not? Or at least an error in the OEM recommended OCI duration? (Presuming you're using OEM spec'd fluids).
I have no idea why we had the valve train issues.
They apparently blamed the oil not getting to the valves (which I don't think any of us believe, including you). If the oil was NOT getting to the valves, that would be a design issue. No OCI duration is going to make that better or worse, presuming they have spec'd a correct fluid.
They thought we were letting the turbo get too hot and it was getting the engine oil too warm.
The turbo should be designed by the OEM to stay cool enough that both it, and the oil, stay in good shape for the recommended OCI (which was 400 hours via the manual). The volume and flow rate of the oil in the turbo should be sufficient to cool the turbo; if not, it is again a design error. Your OCI has nothing to do with this. A turbo that is too hot may cause a sludging factor of the oil, but that is NOT the OCIs fault. It's a design issue in that the cooling capacity of the oil velocity and volume may not be great enough through the turbo. Presuming that all mechanical systems are operating as designed (water cooling, oil pump, etc) the engine should be operating within it's designed load factor. Watching the EGTs is important, but unless that unit is supplied with an EGT gage from the OEM, I don't see how they expect you to monitor it. I would presume that if it DID have an EGT gage from the OEM, then there would be some type of monitoring system for warning if EGTs get too hot. Again, design issue, because if they want you to monitor the EGT, they should provide you with a way to watch it!
With a 5-gallon oil capacity, I think you'd need a turbo about the size of the engine to do that.
5 gallons for 8.4L is a pretty good capacity, and this would go to show that the oil should NOT be sludging due to a low capacity. So if the oil IS sludging, then it's the EGT issue I covered above, or a contributing factor like poor oil return (yet again another potential design issue).
I think I'll shoot for 275 to 300 hours for the next oil change and see how that works out.
Given this UOA, and the other that I presume is similar, I'd say that is a very reasonable decision.
Overall, it seems to me you've thought this through pretty well. I think that some anomolies occured with the engine (the origin of which may have been poor design or manufacture). But your OCI, as I previously stated, likely had nothing to do with it. If you have problems with this engine, and your UOAs are as good as this one, then I have to disagree with others, and say this engine design isn't all that great. You may want to add an EGT gage if you don't have one already.