they are, but but only minimally. when the shoe clearances are too far out but too close to self adjust then the brake pedal will be mushy, taking the car a longer distance to stop....similar to air in the brake lines, that kind of soft pedal feel.
readjusting the shoe clearances will improve brake feel and response.
when the shoe clearances are too far away from the drum the hydraulic system will need to compensate for these clearances until the shoes start to grab. the wheel cylinders use up most of the hydraulic pressure from the master cylinder while the pedal is pressed to push the shoes closer to the drum....once the cylinders take up whatever pressure/fluid it needs, the residual pressure is then transferred to the front calipers.
if the shoes are adjusted closer to the drum then the cylinders won't need as much fluid pressure. if the rear cylinders take up very little pressure to move the shoes then the rest of the pressure will transfer to the front calipers faster and the pedal will be firmer and the car will stop quicker.