Have no idea about the MTOW increase, if possible it will be done later in the program I guess.
Anyway the CS100 is offered with 19k, 21k or 23k lbf engines (84.1, 93.4 and 103.6 kN respectively) while the CS300 is only offered with the 21k and 23k lbf engines.

What is the advantage of PW1521 and 1519 over PW1524?

Anyway: comparing the standard CS100 and base CS300:
they have the exact same MTOW (58 967 kg), same wing, and same engines (PW1524G). The difference is fuselage stretch.

Their take-off distance differs: standard CS100 takes off in 1463 m, base CS300 needs 1524 m.
The causes for difference might be:
the fuselage stretch of CS300 causes extra drag during takeoff run;
the maximum coefficient of lift of CS300 is smaller because of lower rotation angle, limited by the stretched tail, and therefore CS300 needs higher speed to lift off.

Is it known how much the takeoff distance of CSeries is stretched by PW1521 and PW1519 engines?