Just my 2 Cents Opinion, Your 16-35mm. L MK II is already wonderful for Landscape Photography, As long as set F = 5.6 to F= 10.0 ( the Sweet spot F = 8.0) that you will get all sharp / Max. DOF.---But Not try to open MAX. at 2.8, That will be not sharp at the corners of the picture. I use Canon EF 17-40 mm F/ 4.0 L USM almost 10 years and still love this Cheap " L " baby ( you can see the Photos Below), Yes, Most of The " PRO" tell me that If I want to get the Best of Wide Angle Canon Lens that Total sharp from F = 3.5 and All range, I must get Canon TS-E 24 MM. F= 3.5, L MK II, Tilt and Shift Lens, And That will add so many Functions that Normal Wide Angle Lens ca not do. Yes, I agree with the PRO, But Learning Curve are take me too long in leaning/ how to use this Great Lens. BUT, I keep on trying.
Nice to talk with you.