Do you have any idea as to what causes that? I have heard of direct injectors being fired at intake stroke, beginning of compression, or later in compression. If it's later in the compression stroke then mixed fueling would mean less fuel for the direct injector, and that the injector would have less cool down time while the cylinder temperature is high. Injecting more fuel during the end of compression stroke would allow the injector to stay cooler when ignition happens and probably would keep temperature down. The dish guided spray thing seems to suggest injection happening at either beginning of intake stroke or end of compression stroke, the only 2 times you can put fuel in and have the piston near the top.
The problem is I seem to remember being confused about these D4-S articles because there was conflicting information on how the injectors actually operated... At any rate, I have trouble seeing why increased port fueling while keeping the direct injector running the same could cause overheating. More fuel, even through the port, would be expected to lower temperatures across the board
I think that article is talking about why they necessarily HAD to run 100% direct injection at higher speed. I guess that brings a second issue though, forced induction would increase cylinder temperatures considerably and 150C sounds pretty easy to hit if they are relying on the fuel to cool it down during the very short time that injection happens in :O