There are some papers modelling CO2 retention with various sorts of mask but I've not found a really good one.
https://www.oatext.com/Evaluation-of....php#gsc.tab=0
is a model paper showing significant re-breating of expired air with an N95 mask.
https://www.nature.com/articles/s415...raft=marketing
is a paper in children looking at adding a micro fan to N95 masks and showing that in vivo this decreases CO2 rebreathing to nearly normal levels.
Consensus seems to be that FFP3 and other high-protection masks will increase CO2 retention and is is clear they have an effect on performance. However, as we don't even wear them in a medical setting with covid patients unless performing aerosol generating procedures I can't see any justification for aircrew wearing a surgical, N95 or FFP3 mask.