I have an unhealthy interest in this subject and would be very interested in BenH's view point and why he thinks 7's and 8 should be carried together. If GD can break everything with 7.5's then why loose pellet count carrying 6.5's?
CleverSC3 is bang on the money with his take, if you carry big shot for long birds, by definition you will miss more often using them and psyche yourself out anyway. I shoot almost everything with 8's but carry a few 7.5's for going away birds, nowadays not because I believe they best 8's but because I use them as part of my mental routine and an aid in pacing myself.
I would love to spend a day with some boys like Ed, Glen etc testing long crossers and teal with different sizes. I have a sneaking suspicion 8's will win out. It's like this: 45 extra pellets in 8 compared to 7.5, the latter is 4.5% bigger so would arguably carry a minuscule amount more energy at 50 yards. First we have to find out if that will have a meaningful impact on whether an edge on clay will succumb given the other dozen or so factors, even if the answer is yes we are left with the reality that it has less chance to hit the thing in the first place. In the marginal scenario of a single strike, I'd rather have a tiny chance of a break (using 8's) than none (using 7.5's) .
Lets not get into the other argument of misshaped shot etc which we're told saps velocity and thus energy. The 8's have more probability of propelling true undamaged shot since a greater number of those are on the outside close to the barrel walls protecting the main bulk?!