So I stopped playing with the +30% option, since that was a cheat mode, imo.
I switched to the first option (+10% attack) and tried the following fights with Hunter/Druid @NM :+Steam tunnels-sequence. Very doable, difficulty is more or less the same as with the default option, maybe slightly more challenging towards the end of the tunnels, but that's just positive. However, some of the fights started to drag instead of getting quicker because the lizardmen have warriors who spam a row attack that inflicts slow. The additional speed makes it possible for them to spam this attack even more, but since the damage they inflict still isn't really high on that attack bonus nor do the backrow mages have any bonuses in this option, the fight lethality stays the same .. you'll just spend more time waiting to get any hits in with your front row.
+Rowinda's undead felt ok with the +10% option.
+The blades of the moon felt less of a challenge, to be honest... unless the enemy bard starts with Static Lullaby on you. Otherwise the attack and speed bonuses are not enough to compensate against the slight loss in defense and HP ..which are just enough to take those annoying backrow guys down in two swipes each.