Practical recommendations
Ok, enough theory crap. Based on the above data, here’s what I would generally recommend to bodybuilders or athletes who want to put on muscle mass (i.e. all of them).
If you’re above 15% body fat (about 24-27% for women), diet first. If you can get to the 10-12% (19-24%) body fat range or so, I think you’ll be in an overall better position to gain mass. Trying to get super lean will probably end up screwing you in the long run because your body will be primed to put back fat on (and most other physiological systems are screwed up as well) when you get super lean.
After finishing your diet, regardless of how lean you get, take 2 weeks to eat at roughly maintenance calorie levels before starting your mass gaining phase. The reason has to do with the physiological adaptations to dieting described briefly above. Although you can’t reverse all of them short of getting fat again (or fixing the problem pharmaceutically), 2 weeks at maintenance, which by definition should be higher calories than you were eating on your diet, will help to normalize some of them. Leptin, thyroid, SNS output should improve a bit, along with other hormones, putting you in a better place to gain mass without super excessive fat gain. Make sure to get at least 100 grams of carbs/day or more during this phase so that thyroid will come back up.
Only try to add mass/bulk until you hit the top end body fat percentage listed in #1 above. So that’s about 15% body fat for men and 24-27% body fat for women. What this would mean in practice is that you diet to 10-12% body fat for men (22-24% for women), eat at maintenance for two weeks to try and normalize things, and then add mass until you hit 15% body fat for men (22-24% for women) and then diet back down. Over a number of cycles, you should be able to increase your muscle mass while keeping body fat under control