Reel capacity is just a guideline so you can get a ballpark idea of what capacity you'll have at a given line diameter depending on what you're doing with it.
So there are several things to unpack here:
I remember reading that most diving crankbaits are designed to reach their rated depth on 12lb line. Heavier means they don't dive as far, lighter might get you a little more depth.
All else being equal, lighter line will generally allow more casting distance, which is nice.
You're gonna want to consider what the cover/structure situation is where you usually fish.
Remember, too, that traps and cranks aren't the only lures you'll be tying onto that line. Do you generally fish in areas where you can get a big single hook driven home and get the fish yanked out of whatever it darts into after the hookset on 12lb line? That's something that only you can answer.
In any event, with a MH rod and the line you have on it, you'll want to keep what's tied on the other end in mind when you set the hook.
You can fish with exactly what you've got and you'll be successful. Will switching to lighter line make you more successful with small treble-hooked hard baits? Maybe. Would that same change make you less successful with big single hooks? Maybe.
In either case, you will absolutely want to keep what you have tied on in mind when you set the hook, lest you yank those little treble hooks right out of the fish's mouth.
TL;DR:
If you aren't generally fishing areas with lots of brushy cover to have to winch a fish out of on a spinnerbait or chatterbait, I think it'd be worth seriously considering going down to maybe even 12 lb test on your mod-fast to help with sensitivity and give you some fight buffer for the treble hooks. But only you can answer that, bro.
