The answer is to go back to the mechanisms that were in place when to hit modifiers were a thing. That would mean going back to 2E rules where Orks were BS 3 or better, or in 8E parlance BS 4+ and better. Adding to that, as a general rule, any natural of a 1 or 6 always succeeds or fails, regardless of any modifiers (the sole exception to that being armor save modifiers which can cause an auto-fail, but never an auto-success). Also, blast weapons need to have bonus to hit, and have more chances to hit. These "bast weapons of doom" that fire d6 shots...only to roll a 1...which misses...are such utter shite. Worst mechanic of 8E to be sure. There are some horrifically problematic rules aspects to 8E, and simply returning Orks to BS 3...er...BS 4+ is only the tip of the iceberg, because that merely serves to put them functionally on par with 'umies, who pretty much suck all things as it is. So, +1 to Ork BS and a few fundamental changes to the core rules will not just fix Ork shooting, but fix shooting in general. In 2E, shooting modifiers broke down like this:
-1 to hit at long range: a few weapons has this feature; notably shotguns, some pistols, grenade launchers, and primitive weapons in the main.
-2 to hit at long range: very few weapon had this feature.
+1 to hit at short range: most common weapons (i.e. lasgun, autogun, bolter, shuriken catapult, etc.) had this feature.
+2 to hit at short range: some weapons had this feature; notably most pistols.
Other penalties to hit, other than those based on the weapon being used, included:
-1 to hit for soft cover (brush, woods. light fencing, etc.)
-2 to hit for hard cover (rocks, buildings, trenches, etc.)
-1 to hit for special circumstances; includes things like chameleon scales of a lictor/cameleoline cloaks of beakie/panzee scouts (they only got the bonus if the unit didn't move), etc.
-1 to hit short range/-2 to hit long range for units that had the Stealth ability, like Striking Scorpion Exarchs/Kharandras.
So, the maximum penalty to hit was usually limited to -3, which could be offset by +1 by using one of the more common infantry weapons in the game and being at short range. Seeing as no unit meant to shoot had a BS of worse than 3, that meant most units would hit on a 6 or better when firing at most anything. Even still, the rules accounted for needing a 7 or higher to hit, requiring a natural 6 followed by another roll of 4+ if a 7 were needed, 5+ for an 8, and 6 for a 9. If you needed a 10+ to hit, well now you finally just can't hit.
This is how the 2E Ork BS stats would translate to 8E:
Snotling: BS 5+
Gretchin/Ork/Warp'ead/all vehicles & artillery: BS 4+
Gretchin Champion/Nob/Runtherd/Painboy/Mek/Weirdboy/Kommando: BS 3+
Big Boss/Kaptin: BS 2+
Warboss/Nazdreg/Ghazgkhul: BS 1+
As 1 always misses, BS 1+ would represent being able to offset a -1 penalty to hit and still hit on a 2+ (this could similarly be applied to armor and save modifiers).
Yes, Orks could shoot the $#@! out of stuff back in 2E. They could do it through sheer volume of fire/blast/template usage from 3E-7E, but now they (and some others) just plain suck arse; the more expensive the unit, the more arse it sucks even.
So, GW went and added all of the to hit penalties back in, but then didn't have enough sense to pull their heads out of the arses and also add back in the bonuses to hit that most weapons had, and screwed over how blast weapons work. And so there is a big problem that really can't be fixed even by just giving Orks +1 to their BS stat, though it would be a start. 8E is sort of fundamentally a $#@!-up edition. There are many things about 8E that I think are improvements over 7E, but I also think that in 8E GW has screwed up what is arguably the most fundamental phase of the game, the Shooting Phase, which is horribly bad. We'll probably get 8.5E soon enough...or just a slew of added "get-you-by" half-arse rules to band-aid things until a true 9E comes out.