As some others have said, the increase isn't linear by percentage but by raw numbers. If it was linear by percentage that'd be exponential growth. About 20 DPS is common for every 3 weapon damage. So the 5 weapon damage and 40 DPS increase is normal. Back in the day going from relic to zenith was something crazy like 20%. This is also because as the weapons grow so do all of our other gear.

Also, x5 weapons usually had 2 wd over x0 weapons, the last tier had only 1 wd.