Small gargants counting as 15 models instead of 10 is also thing to remember during objective game. Often enemies leave some small size units to protect their objectives. I have often used retreat from combat with another unit with mancrushers to just tag an objective and steal it from 10 man units + heroes sitting on them. This wouldn't be possible with other tribes.
In battletome it says "the result you would have made is automatically substituted with the result shown on the Destiny Dice". It still is unmodified roll. Modified roll is one affected by any skills that adds/substracts/multiplies/divides it. Like "increase hit roll by x" or "substracy y from Bravery".