Is the high-framerate crop mode in the Red Scarlet camera practical?
This question is pretty obsolete by now. The new SCARLET-W cameras support 5K WS (2.4:1) video at 60 fps, 4K WS at 150 fps, and 2K WS at 300 fps.  Matching field of view is a simple matter of mathematics, not magic.  If you want to match a 50mm lens on a 6K RED camera using a 3K crop, a 25mm lens will do well.  And the price of the SCARLET-W is now below what the price of the original SCARLET was in 2011, so you get a lot more camera, as well as more sensor, for less money.

It is relatively well-known that when shooting slow-motion footage, it is often useful to use an even lower compression level than for normal motion footage, which is unfortunate.  But the DRAGON sensor, in a WEAPON body, makes SCARLET-W 10:1 as good as SCARLET DRAGON 8:1, which in turn has been estimated to be as good as SCARLET MX 6:1.  You will want to use special noise reduction techniques if you shoot at the limit of the SCARLET-W, but if you shoot 4K WS at 60 fps, the 6:1 REDCODE is really quite good.  And 5K WS at 60 fps REDCODE 9:1 is also not bad, either.