I am not convinced in the benefit of a flat-plane (other than the sound). If you nix the counter weights, you will still have vibration issues. If you leave the counter weights, I fail to see the benefit.
If you want a motor to rev - or rev quickly, counter weights are not the issue. With good parts, RPM is generally limited by valve train. Revving quickly is limited by all the "crap" spinning.
We have a couple of heavily restricted 305 sprint car motors. These have a 48lb crank (per rules), so full counter weights. Same stroke as a 350 (smaller bore). So not a design that lends it self to high RPM. These motors run 7200-7400rpm lap after lap and we occasionally hit 8200 (not recommended). With no balancer, flywheel, clutch, transmission, altenator to spin, they will go from idle to 120mph wheel speed about as fast as you can move the accelerator pedal. Keep in mind these are heavily restricted motors (less than 500hp). Not the monster motors in some classes.
If you really want high RPMs, go for a DOHC, 4V/Cylinder motor with GOOD internals and well built.
If you want quick revs, go for a small diameter clutch, minimal flywheel, nix all the accessories except the alternator. Run an electric water pump. And of course live with the difficulty of getting off the line...
Then the expense of a flat plane, along with custom cam and aftermarket ECU. Big bucks that could be used to build a much better motor (and 180 degree exhaust).