Take note that it is the max die result that is modified, not the result of the roll, so the effect of overloading by 1 is the largest individually, but still manageable by good leaders.
Example: no overload, leader skill 6, the test is D(10)<=6, the chance of success 6/10=60%
1 over limit, D(11) is used instead of D(10), the chance of success is 6/11=54.5%
5 over limit, D(15) is used, the chance of success is 6/15=40%
To maximize command effect I think it's better to never overload, even fronts. Just keep all the extra armies under STAVKA (which is never overloaded) and stick to 4 armies per front (5 when rifle corps will reduce load).