This entire issue is a pandoras box
First, as mentioned, there is never enough information given with specifications to start with. CFM rating, at what point in the collection chain? New bags? 50% clogged bags? Slightly clogged Hepa filters? If with hose use, what diameter hose, what length hose? The list goes on..... In the end, for a product of this relatively low price point, (vs. products that offer highly detailed specificaitons) offering any CFM rating is about all ya can expect. It's a lot better than the Borg collectors claiming 5 hp motors... which if it did have a 5hp motor, the consumer could not lift the vac in his vehicle without a hernia.
A few corrections.... there is an inverse relationship of air velocity and air volume, however, its not perfectly linear in cases like this. As the inside of the hoses surface coefficient is a major variable to friction. And friction is air movements (regardless whether velocity or volume) worst nightmare. As a general rule, thinner diameter hoses create greater friction, which reduces total air volume. OTOH, thinner hoses increase velocity (FPM) while reducing total volume (CFM), and visa versa as well.
I have spent many years testing air systems, and of course tested the Festools for fun. And its not worth mentioning the results as I have no desire to stir up a bees nest, I will offer these few findings...
The 50mm hose vs. the 27mm hose at 3.5m length will allow almost 2x the total volume at the point of collection. The significance of this is, for floor sweep, the 50mm hose is ideal, assuming you make a mess on the floor often
Remember, everyone uses these vacs for different reasons. The 36mm hose is a good mix of ease-of-use and limiting air volume losses. The 27mm hose is ideal for low volume applications, such as dominoes and sanding. In both of these operations, the amount of air volume required is so low, the value of the nearly weightless 27mm hose over the heavy n bulky 36 & 50mm hose is worth it, assuming you do these operations a lot. For example, with the ETS sanders, the 36 or 50mm hose would overwhelm the benefits of the light weight of these finishing sanders. Ya end up fighting the hose force way more than the sanders force / weight.
anyway, my point is, regardless of the specifications, what matters most is, how does the vac perform. And by performance, there is more issues than just CFM. This is where Festool vacs are so impressive, they have a tremendous lineup of well made hoses to always fit the right hose to the job. The vacs are very portable, so the shorter the hose, the higher the air volume. Case in point, by combining all my hoses, the festool CFM was reduced to about 15 CFM, this demonstrates the biggest culprit of suction at the point of end use, - hose length.
If you want a Vac with more suction, such as some of the Borg vacs for $99. However, don't think the higher suction comes at no cost. The price you pay is the volume of waste, and particle size of the waste, re entering the workplace out the exhaust. This is the balancing act that Festool engineers did an excellent job of with their Vacs. They valued air quality first with their design, and let the CFM become a secondary issue. However, they did not compromise CFM, as they maximized filter area in the design, which provides the most efficient possible filtration possible within the physical constraint sizes of the Vac dimensions. So anyone making a Vac choice may want to consider the BIG picture vs. getting caught up in one specification. Of course, there are always exceptions...if you are working outside in fresh air, the HEPA filtration is of less value vs. working in confined spaces, and in those cases, a Borg Vac will work excellent. For larger machines, no portable 110V vac will be adequate.... so its the ol adage, horses for courses that applies with vacs...
