If you want to reduce it to -5/+5, it is because you don't understand math. That combo will never be beneficial
TO reduce to -5/+8 is better, but likely also worthless in the vast majority of situations.
You really need to keep the X/2X ratio to make it ever worthwhile. So either limiting it to 1/action, or switching to something like -4/+8 or -3/+6
Keep in mind that the more damage you do, the less likely the -5/+10 is worthwhile.
The less damage you do, the more likely it will be beneficial. But that also means that it is somewhat self correcting, it helps low damage more than high damage.
There is a curve to how much it helps. For example, a +8 to hit with a 2D6+4 GWF/GWM fighter means it causes more damage at AC17, and causes less damage at AC18. But either way only a few percent. You have to get down to AC13 before you get past a 20% damage boost.
Which means if you limit it to 1/action, even at AC13 most classes are only getting an 11% damage boost, and if you have 3 attacks, only a 7% damage boost.