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Again, probably revs (namely acceleration of a rotating mass).
Audi's 4.2 liter V8 used in the RS 4, RS 5 and R8 has pistons that at 8000 RPM or so travel at near F1 speeds, but that's over a huge stroke length and half the RPM of a V10 or V8 F1 engine of the past 20 years of so (in specific 2004-06 when the Audi RS V8 was developed).
And even it had a crossplane crank. The issue with the flatplane crank isn't RPM itself, but speeding up/slowing down. Flatplane cranks don't have the counterweights like crossplane cranks have. Lighter crank equals faster spin up. Flatplane cranks, though, do have a disadvantage in terms of high RPM vibrations, usually above like 8000 RPM and it gets worse the higher the RPM without good crank balancing.
Over-square cylinder dimensions allow for more RPM due to a shorter stroke distance (less distance for the piston to travel). A flatplane crank in a V8 will allow (in theory) faster spin up/slow down times.
Of course, this does matter for Toyota, who have a flat crank V8. For the Audi and Peugeot V12s, and Audi V10 and V6 diesels, they have their own crank designs that have no major variations from that would be expected from such engines.
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