Well, Sylar might be able to use stolen powers at an advanced level, due to the nature of his original power; 'intuitive aptitude'. His power allows him to understand how things work almost immediately. For instance, he has a very high mastery over telekinesis, way more than Brian Davis (the guy he stole it from). So I think Sylar would be able to used the so called 'advanced levels' of a power, because he would understand how it works.

Peter, on the other hand would probably have to practice extensively, and maybe then it wouldn't make a difference, unless he comes to understand the nature of the powers. The thing about Peter is, that his ability is built on emotion, which can be unpredictable, and thusly could affect his control. Sylar's is built on logic, and therefore is more stable, which would allow him to build on what he knows more easily than Peter.

P.S. And its "Parkman"