D2 is a "strength" day so my thoughts are that adding more sets skews the focus towards endurance rather than strength. The first thing to ask yourself is are you taking too long a rest time between sets? Maybe shorten is up a bit. Second, and I know it goes against the simple in "simplefit", but maybe add some resistance to the D2 exercises? I've always wanted to try resistance bands but haven't been able to justify the cost to my wife. If you can swing those, you could use them in squats and pushups, and hug a weight to your chest for the squats (different sized patio stones or something similar make a good cheap substitute for barbell plates). If you can't get the exercise bands then you can rig up something to hang around your waist for the chinups.