A casual look at the issue would indicate their has to be at least some reduction in performance, but is it significant?
I doubt it very much. (But could be wrong)
The reason is as I mentioned just above, they are an on-demand type mask and so don't have to hold in any pressure as such. As soon as you breath in, the regulator senses a slight drop in pressure (don't need much) and starts pumping in oxygen in either diluted or 100% flow depending on what you've selected.
As long as there's a bit of a seal on the edges of the mask it'll work fine. I can see that perhaps a tiny little bit of smoke
might get sucked into the mask if the seal was particularly bad but probably not enough to cause discomfort.