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at 120mph once (1200 sedan CA18DE non-turbo) and I found the wipers were almost useless
A 1200 sedan? Surely you mean 120 kph (just kidding, I know your car is far more powerful than a stocker).
Isn't that what those wiper arms with the airfoil are for? High speed usage. The airfoil is supposed to keep the wiper arm against the windscreen. I never tried them myself (that is, not at high speed).
On the other hand, the 1200 wipers are inadequate at normal speeds in heavy rain. I would think other car's blades (or aftermarket) would fit on the splined posts. A longer blade might help. But I think the motor might need a little more juice.
Variable speed and intermittent is possible with a little add-on box, like JC Whitney sells. To do intermittent, you either need to use the manual method (turn it off and on every so often) or use an electronic circuit. Some older vehicles had mechanical intermittent IIRC, but this wouldn't be easy to make. I seem to recall a cam-based system on some older cars. (I used to rebuild and test wiper motors, but that was 20 years ago). It might be pretty easy to swap in a different OEM unit (say from some newer Nissan model).