One of my professional friends who is head brewer at a local place said, even with 6.5 gallon carboys, I should still use a blow-off tube and, in some cases, you will lose some beer. I did a 4.5 gallon batch single hop Imperial IPA (around 9-10%) and, in the 6.5 gallon carboy, I still had some insane krausen that took up all head space and started going into the airlock so I switched to a blow-off. I lost less than I did before when I lost a full gallon of beer..same recipe in the 5 gallon carboy but still lost some so nothing is perfect.
I'm looking at those big conicals but $35 for carboy versus $1200 for a conical. I don't mind losing a gallon of beer from time to time for the cost savings.