I haven't actually seen either GWF or SS feats in play, so I am commenting only on the math.
+10 damage is good, probably around doubling your damage on a hit (for argument's sake I am assuming 12 or 11 damage on a hit normally), but it comes with a 25% drop in accuracy.
Players with those feats usually save the big damage attacks for low AC monsters, even at high levels.
I think that in general the idea that a -5 to a d20 roll equates to -25% probability is misleading. I would rather think of it as the following:
Code:
Old roll to hit Old prob New roll to hit New Prob Hit rate difference
1 or less 95% 7 or less 70% or more -21% or less
2 95% 7 70% -26%
3 90% 8 65% -28%
4 85% 9 60% -29%
5 80% 10 55% -31%
6 75% 11 50% -33%
7 70% 12 45% -36%
8 65% 13 40% -38%
9 60% 14 35% -42%
10 55% 15 30% -46%
11 50% 16 25% -50%
12 45% 17 20% -56%
13 40% 18 15% -63%
14 35% 19 10% -71%
15 30% 20 5% -83%
16 25% 20 5% -80%
17 20% 20 5% -75%
18 15% 20 5% -67%
19 10% 20 5% -50%
20 or more 5% 20 5% no change
Of course my math may be wrong, but what I see is that in general the
hit rate (i.e. how often are you actually going to hit the target) is always decreased by more than 25%
except against extremely easy targets which you would normally always hit unless you roll a natural 1.
For example, if you would normally hit on a natural 11 or more (hence your hit probability is 50%), if you take -5 then you would hit on a natural 16 or more (probability is 25%). The amount of successful attacks is therefore
halved, not reduced to 3/4, so in general the loss is quite a lot more significant.
If the table above is correct, then taking -5 in exchange for bonus damage is more convenient against easy-to-hit but high-HP monsters, which in fact also makes sense narratively.
However, the "automatic hit on natural 20" rules creates the anomaly in the last table rows: since you always have at least 5% chance to hit, taking -5 is also
the least convenient when you hit on a natural 15 or more, then the cost progressively drops. It might therefore makes sense to take -5 when you would normally hit on a natural 19 or more (because your hit rate drops by half, which is large but doable), and it is actually a no-brainer when you only hit on a natural 20 (because with -5 you still need the same natural 20 to hit).