I think a possibly better solution would be to give Slayers an 'axes of fury' rule, where every wound by a model generates another attack. The reason I say this is that it is universally useful vs monsters, MI and hordes and provides a benefit even when 'slayer axes' don't benefit them, like low toughness units. This makes slayers good, not only vs monsters but also gives dwarves a unit that works well against hordes. (In my picture of 8th edition dwarves, they would lose the s+ runes on grudge throwers so would need another way to deal with massed infantry). I would not increase the price and would possibly allow a slayer unit for every slayer character as a core choice to try to get away from the great weapons and guns that Dorvz are locked into right now.
Better runic standards would also definately be needed for the army in general and these could help slayers too.