The core is 5mm thickness + 2x 1mm skins. I can't say strictly the smaller cell size causes much more strength and rigidity, probably yes a little bit, but I can say much more load resistance and impact resistance...and a little bit weight increasing of course. At the panel above the impact resistance was the crucial, I used aluminium skins instead laminated. The AL price was much more lower than kevlar.
From my point of view, on R1000 no issues with honeycomb cell size 5-10mm. It's up to you, depends on what you want to have...