You may spot a number of other alterations in these pictures, but for now I will talk just about the difference in positioning. Namely, the bike now has a longer stem (8 cm, instead of 6 cm as previously). The handlebars have been positioned slightly lower as well.
It's important to stress that I would not have found this position comfortable a year ago, so making these changes is not so much "admitting a mistake," as altering the bicycle's set-up to reflect my changing skill level and preferences. I very much find this position comfortable now - comfortable and fast.
We rode to Lexington, MA to try some hill intervals. I did not attach a saddlebag, so that I could get a sense of how the bike itself performs. And it performed well - albeit differently from the Seven.
Here is my amateur description of what happens on the two bikes when I cycle uphill: The Seven allows me to very easily "attack" a hill - to cycle up it at a fairly high speed, until I run out of steam toward the end and am forced to slow down. In comparison, the Rivendell does not like to "attack" and forces me to slow down earlier. In its previous state, this slowing down happened almost immediately and drastically. Now it happens half way up, and the speed drop is considerably smaller. Cycling on flats, the new positioning makes the Rivendell faster to accelerate than previously. Not as fast as a racing bike of course, but I no longer feel as if I am sitting "behind the bike" unable use my weight efficiently.
Once I attach bags, that will no doubt slow it down some. But I have a strong feeling that the change in performance is due to the forward placement of my weight more than anything else - when I ride the bike now it feels inherently different - more responsive and more efficient. I am not trying to convert the Rivendell to a Seven and I am not considering taking it on paceline rides. I want to keep the wide tires, the fenders, the rack, the bags, the lighting - there is no compromise possible on that end, as these features are extremely useful to me. But the Seven's positioning (and my enjoyment of that positioning) has most definitely affected my expectations in terms of performance, and I don't think I can go back to short stems and seatposts with generous setback. Not that there is any reason I should - It's all about what you are comfortable with at any given point in time.