Quickly chiming in since I spent months pixel peeping these for my own R5. To answer your question directly, the Canon RF 15-35mm f/2.8L IS USM is actually the sharper lens in the corners, especially at the wide end. I brought both to Glacier last summer to compare them for some 40-inch prints. Honestly, the f/4 version relies way more on software corrections for the heavy distortion at 14mm. When you stretch those pixels back in post, you lose some of that fine edge resolution. Its pretty noticeable if you look close. Technical bits to consider:

• The Canon RF 15-35mm f/2.8L IS USM has a better optical design for clean margins.
• The Canon RF 14-35mm f/4L IS USM struggles with corner smearing at the widest focal lengths because of that digital stretching.
• The f/2.8 isnt just about speed, its about the superior glass. For big Banff prints, go with the f/2.8. Its heavier tho... but worth it for the crispness.

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Regarding what #2 said about "This ^"

• I have to disagree a bit based on my own experience. I actually found the performance on both of those lenses to be somewhat disappointing given the hefty price tags. I took the f/2.8 version out for a mountain trip last fall and honestly, I was pretty let down by the results on my R5.
• corner smearing was still visible even when I stopped down to f/11
• the copy I had exhibited some weird decentering issues at the wide end
• price-to-performance ratio just isnt there for serious landscape work I ended up moving away from zooms for my wide-angle work because the technical trade-offs were just too much to handle for big prints. It's frustrating when you spend that much and still see soft edges. I switched to a prime that cost way less and it totally outshines the Canon RF 15-35mm f2.8L IS USM in terms of pure resolving power. Just my two cents since you're dropping a lot of cash for Banff... maybe look at some primes before committing to the zoom life.