There's certainly some small degree of religion about this topic depending on who you ask , and I've seen some of the dyno tests out there. But unfortunately many of those have a couple of other variables in play at the same time such as exhaust x-overs, etc. that make it hard to ascertain what is contributing or impeding performance where.   So, for those that have ridden, tuned, or have bikes where you've tried modifying the stock airbox with something like Fast By Ferraci's lid eliminator