Important clarifications:
- GWF, or Great Weapon Fighting, is the fighting style. GWM, or Great Weapon Master, is the feat. It's important to keep these two acronyms separate.
- Spellcasting power is at best tertiary to this discussion. There is a caster/martial divide, though it is much smaller in 5e. However this topic is talking about the power that this feat gives some martial characters. That inceased power isn't really relevant to the topic of the caster/martial divide and to use this feat as a "fix" to that divide is incredibly problematic as it does nothing for many martial builds. At best this strategy would further weaken classes like Monk, Rogue, Ranger, or TWF.
Ok, so with those things out of the way lets consider the
actual math:
Math provided on google spreadsheet:
DPR of Classes
So for a 5th level barbarian any AC up to AC 18 the Barbarian is better off having taken GWM and using -5/+10. Assuming a barbarian fights an equal distribution of enemies from CR 2 to CR 8 from every officially published WotC book the average enemy's AC will be 14.4. This aligns closely with the DMG's recommendation of 15 AC for a CR 5 enemy. So the AC of an enemy would have to be much higher than average to have a negative impact on this choice.
Using that 14.4 a 5th level GWM Barbarian is averaging 36.7 DPR while a 5th level Barbarian who took 1 strength instead is doing 23.3 DPR. The difference is massive:
57% more damage.
Sure there are cases where a barbarian isn't going to want to use -5/+10, but as you can see in the graph above the Barbarian is still better off taking GWM and not using -5/+10 than taking +1 str.
Let me make that part clear:
The cleave part of GWM is better than +1 strength modifier in terms of damage.
So even if we ignore -5/+10, GWM is an incredibly powerful feat. Add in -5/+10 and it's definitely out of the expected range for damage - especially for classes with easy access to advantage like Barbarian, OoV Paladin, Fighter (BM w/ trip).
Does it make martial classes equivalent to casters in versatility? No.
Does it make certain martial classes do far more damage than expected compared to their martial counterparts? Yes.
It's overpowered.