Maybe it's nothing but...
Shaving Johhny Rocco -> The Untouchables (Al Capone shaving scene)
"I bet you look in the mirror with that gun and try to act mean" (or something like that) -> Taxi Driver
Martin Scorsese does like to make references to movies that have influenced him.
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Not the first time a scene was filched from a previous film. A popular one is the girl running to her beau and the hat falls from her head. This is quite often done in a crowd (train station, bus depot, downtown, and others). Cliche, but very effective. It is done probably because it just would not look right with the hat on at the tender moment of joining, and for her to actually take her hat off is a bit too "mechanical" for the romantic effect sought. Back to the shaving scene. Johnny Rocco and Al Capone are very much alike as far as image and reputation. Giving them a common scene makes sense and in this case works very well. But, then we are talking about Little Caesar and DiNero.
- JKHolman