Ikea recommend 3.5kg (8lb) maximum per shelf. The existing shelf glass is about 4mm, so going to 6mm will certainly improve loading. I won't be adding any more to each shelf display, so having that extra "buffer" will make me feel easier.
I am pretty sure that Ikea allow some leeway in that maximum and discussions on large popculture figure collector forums say much the same. Glass shelf failure seems to occur when a large centre weight (such as a large stature in the figure collectors example) is placed on one of the shelves. At least my display loading on the shelves is dispersed. Still, even with that, I am going to put in thicker shelves.