A fuel system includes a fuel injector having a split check assembly with a control piece, an outlet piece, and a check sleeve. A trapped volume is formed between the control piece and the outlet piece within the check sleeve, to hydraulically couple the control piece to the outlet piece. A starting rate shape clearance fluidly connects the trapped volume to a fuel cavity and is formed between the check sleeve and one of the control piece or outlet piece received therein, and modulates a starting rate shape of fuel injection from the fuel injector. Related methodology is disclosed.