Sounds very much like a young version of Lukaku then doesn't he?
The difference is Lukaku is a more natural physical presence and while he needed to, and still has room to improve, around being a top focal point for the team he has the physical ability to be a dominant forward and when he has games where he is on top form he is a world class all round forward and that was true when he was 19/20 as well.
Iheanacho may well only be a couple of inches smaller than rom but he isnt as an imposing figure. And the criticism I have sometimes with rom would be he doesnt make the most of situations when the ball goes into him as in should hold it better rather than attempt a quick lay off, or should win a free kick rather than try and hold up an impossible ball. With Iheanacho when watching him it was that he wasnt getting balls into him to lose, he wasnt giving good options for the midfield to play it to him he just wasnt involved which is crucial if you are the lone striker.
The other big difference is the type of goals they both score or have scored so far. We know rom this season especially, but over his career can score left foot, right foot, headers, goals from outside the box, free kicks. If he gets a chance on his left you dont think he will miss, if has a chance with his head you expect him to score 70% of the time say and if it was on his right it might go down to 50% but the reason he scores so consistently is it doesnt really matter how the chances to fall to him. With Iheanacho it's something like 90% of his goals have been with his left foot, only has 1 header and 1 right footed goal. I know rom is better than nearly everyone at spreading how he gets his goals and not everyone is strong on all but as a lone forward it's a big weakness if you need a chances to fall in one manner.
To add roms spread for his 84 PL goals are 41 left foot, 24 right foot and 18 headers.