No you really can't with enough practice make a mildly aggressive razor do what a very aggressive razor can do. The shave from the milder one will not be as close or as long lasting as the aggressive one. If you have your shaving technique down pat, have a good lather and taking your time using a very aggressive DE it is very possible even at a 9 setting on an adjustable. It won't be a shave where you can get lax though. That is where an adjustable shines for me. If I want ultra close it is set at 9 or if I just want to cruise a 6 setting will do. For the same reason an adjustable is a good DE for the beginner as he can grow into it by dialing it up as his skill level permits.
Bob