Scaling caps exist, but are somewhere above the recommended light level of the target. Exactly how weapon damage and total light level interact with damage scaling is fuzzy. Similar for damage taken. And how all of this interacts with character level differences (not light level, character level) is even fuzzier. But I'm pretty sure that hardmode Wrath of the Machine has a light level cap that isn't currently reachable.
I tested in the Plaguelands patrol with all 400 light level gear, and both a light level 400 and 365 1000-Yard Stare. Plaguelands is recommended light 320.
400 Light Level 1000-Yard Stare did 43325 damage
365 Light Level 1000-Yard Stare did 34209 damage
Weapon level matters. Increasing weapon attack from 45 points above the recommended amount to 80 points above the recommended amount gave a 26% damage increase. And the cap may have been somewhere before the full 80, but it's definitely more than 45 light levels above the recommended. That HM WotM, at 380 recommended light, has a cap that kicks in before light level 400 is simply preposterous.
Tests against low level enemies will be confounded by character level effects. Tests with very large light level differences will be confounded by the light level cap. Technically this isn't definitive because it's not in HM WotM itself. But a test in the Plaguelands has more similarity to Wrath of the Machine than tests in year 1 story content.
TL;DR: get to light level 400, because it matters.