Yep! That's right tshile...For me, I don't sweat the 'headspace' thing. Supposedly, the less exposure to air (especially in secondary) the better, but I mostly just avoid shaking the carboys at any point. The better reason for a 6 gallon primary is just that this allows you to drain off 5 gallons into secondary and leave the 'drub' (the yeast, hops, and any grain that worked it's way into primary) behind. You'll be surprised at how much of the 6 gallons you can siphon off without getting any solids into your secondary. You can still dry hop in a 5 gallon - depending on how you do it, it's a non-issue. I have a little strainer thing that hangs down into secondary from a chain, and have also used new panty hose for that which works great.