I've been fermenting this way for at least 4 years now, and I can say from my own brewing that attenuation has never been an issue. In addition when I first transitioned from bucket fermenters to kegs I brewed many of the same recipes I had done in the past, and there was no significant difference in attenuation between batches. Similarly, all of my ferments are temp controlled, and my process is standardized for yeast pitching and aeration. IOW, a fairly controlled set of parameters existed outside of the change in fermenting vessels.
Bottom line, perhaps the overall volume(5-6 gallons) and relatively minor decrease in size does not have as great of an impact in this scale; I am not disputing the science, but based on the equipment and observation techniques available to us outside of a well equipped lab, I feel very comfortable fermenting in kegs (originally corners but the last couple of years 1/6 barrel sankeys), and see no reason to scare people off from using them either.
There are many factors that can impact your fermentation, but fermenter size is not one I would concern myself with too much at this level- temp control, proper yeast pitching rates, and oxygenation of wort are the biggest factors I have experienced over the years. If those are dialed in and you notice a perceptible difference in your beer post fermentation, then maybe switch back to a bucket or carboy. I still believe though that even if you notice a measured difference in attenuation, the perception of the final beer is what matters.