I know how you feel. I just switched to Texas Method about 3 weeks ago. It's the original Madcow.
The theory behind these intermediate programs is that you've programmed the load on the bar such that this workout sufficiently disrupts homeostasis. Meaning that, if you've chosen the correct weight, you don't need to do more volume to trigger an adaptation.
Mark Rippetoe's "intermediate" book, "Practical Programming", describes the Texas method in detail in chapter 7. He's also got a quick rundown of the same thing on T-Nation
From the link,
Assistance Work: If it were up to me, I'd limit any assistance
exercises to some brief arm work on Monday. I'd also limit any
excessive weekend frivolity that might affect the workout... The Texas Method is still very simple in terms of the number of exercises.
The theory behind Texas and Madcow are the same. They were invented by the same people. You can see that Rippetoe takes a clear stance against more volume. If it's too easy to complete the workout, he would suggest upping the weight. Perhaps resting more time between sets on your volume day would allow you to do this. (He recommends "no more than 8-10 minutes" between work sets. That's a huge rest period.) The truth for me is that I can't even do 5x5 90% 1RM. The volume day completely crushes me at 80% 1RM.
The main thing is that you to need to be sufficiently recovered from one Monday to the next so that you can increase the weight. If you can't do that, then you aren't doing the program. If you can do that, then you are following the program. If you can do that even with additional assistance exercises, then I would suggest dropping the assistance exercises and upping the weight. Even for badass lifters, there is definitely some amount of weight X that they can put on that bar such that they will be so taxed that the idea of assistance work doesn't come up. If a lifter is at a point where they are unable to fully tax themselves with a properly weighted 5x5 across, then they're probably still better served with a linear progression program.