Export 16k not (always) working

Hi everyone ! 

I'm working on different brushed metal for watch advertising and in some cases, especially for close-ups, a 16k version would be great. 
Theoretically, Designer can generate outputs in 16k but in my case, it wont. Too render in 16k, I simply select my graph and increase the output size (I have also increase the memory budget, the cooking size limit and the GPU cache limit). And it seems that it struggles on the same nodes everytime, which are the splatter circular and the tile generator. 
Does anyone have an idea ? Or is there known limits like in instance amounts on those nodes that makes them enable to render in 16k ? 

Thanks to anyone who will help me !