Agreed, you'll probably need the "Boeing push" more than ever - but Boeing recommend F40 for ditching. If you stay clean (or clean-ish), the engines will strike the water first, but the speed is now some 30-40 kts higher than the speed the ditching criteria were build around.
I see it as two seperte problems: Forward speed (impact forces and severity of any loss of control) and pitch-down-moment from dragging the aeroplane through the water (since a high pitch-down-force could break the fuselage forward of the wing box). The pitch-down-moment will occur no matter what, only we're now talking slightly less moment arm (only engines, no flaps worth mentioning), but at a higher speed. How much difference would that generate from the standard ditching scenario, where you hve a greater pitch-down-arm (flaps and engines), but the speed is lower?
I think the two above scenarios more or less balance eachother out - i.e. we end up with roughly the same pitch moment, but with less forward speed and thus energy to dissipate - or, if the water is less smooth than the Hudson and it's not your day, to take into the cartwheel.
That's more or less my reasoning for staying with F40 - but have no hard testing data or other evidence to back that up
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