When an exit bag or hood fills up, the gas will fill until the positive pressure inside is great enough to purge the gas through the bottom. In my opinion ( definitely open to being corrected ), the force of pressure isn't necessarily any drastically different at the higher lpm . So it's not like 25-30-40 lpm will have more force to blow of the hood or anything , you'll just fill up quicker and reach that necessary bottom purging force alot quicker just get faster purging of gas .
Hope that makes sense .
On my tests at 15 lpm and 30 lpm , the bag fills up like a ballon pretty much the same, just at higher lpm it purges way more air out the bottom, quicker, and fills up quicker .
The advantages for this in my opinion , is that higher lpm purges cO2 quicker , making air hunger possibilities and hippocampus alarm less likely . The disadvantages are obvious, if you are limited supply on gas . This is why they generally say 15 lpm is sufficient , especially when alot of people exiting are older and weaker and can't lift 60-100 cf tanks .
So final exit recommends 15 lpm with a 20 cf tank which is pretty small ( about 20-27 minutes of flow at that lpm )
For people like me that are having trouble getting this over with and have SI issues , I sm a candidate for higher flow and fancier rigs .
15 lpm is totally fine , I currently am in the school of thought that higher can only be better , with no disadvantage other then more gas loss