I've run both the Marc 1 and the Corsa from ZG, in addition to the stock unit.
The stock unit performs admirably. While ultimately not up to the task of displacing large amounts of air, it works well for day-to-day riding.
The Marc 1, while good looking, was essentially useless for me. The constant wind blast made sustained high-speed riding in anything other than a centered tuck tiresome.
The Corsa is where it's at, for me at least. I'm 5'10" and 240, so getting all my bulk behind the screen is tough. With the Corsa I get coverage even with a large tank bag. You still need a bit of tuck to get completely out of the wind, but standard riding position is comfortable for 120mph+ sustained touring.
Oddly, one of the biggest influences for me is the gear I wear. When I'm in my touring getup (Alpinestars Bogota) I find that I have more turbulence behind my head. The buffeting is nearly intolerable. Once I switch to my street leathers with a built-in speed hump, the buffeting is eliminated and the entire flow over the bike is considerably smoothed. Obviously having a speed hump on the jacket isn't for everyone, but it's something to consider if you like banging out 200mph runs.
2016 ZX-14R SE ABS (Harambike) - Brock's Ti Dual CT, 2WDW Flash, double bubble windscreen, Givi Tanklock 15L with 12v outlet, SW Motech Quick-Lock Evo Contour rack with Givi V35 bags, SW Motech tail mount with Coocase Wizard top box, KAOKO cruise control, fender eliminator, Vortex rearsets, Vortex V3 fuel cap, Vortex spools, Stompgrips, PSR SBK passenger pegs, Helibars risers, R&G radiator guard, frame plugs, Starrotors 55w 6K HIDs, Brembo MC/Rotors/Calipers, braided steel brake lines. 204hp/120tq
2013 ZX14R (Mjölnir)- Brock's Ti CT Meg, 2WDW Flash. 202hp 119tq. RIP
2008 Kawasaki ZX-10R trackfighter - Yoshimura R55, PCV, Servo Eliminator, Traxxion AK20 Cartridges, Penske Triple Clicker shock, EBC HH pads, Pirelli Superbike SC2 slicks, custom subframe, RSV4 tail, Ignition relocate, KX450F number plate, Apex adjustable clipons, CRG levers, Lever guards, Vortex rearsets, Stomp Grips, XT Lap Timer. 188hp, and not as much tq as the 14R!