I have had both a rev box and a +6 key and the +4 I've been running for over a year.
The key offers much better gains, is less than 1/2 the price, and doesn't sacrifice reliability when set up correctly. I ran a rev box for a couple months, re-installed the stocker and sold the rev box.
2000-440ex Stroker--Fan Cooled
Houser +1 A-Arms, Elka Triple rates, Laegers +1 1/4 Swingarm, G-Force Axle w/Durablue Anti-Fade Locknut, 14/36 Gearing, DID X-Ring Chain, C&D Revalved Rear, Maier Silver Plastic, Steel Grey PCed Frame, SPAL Fan, AC Bumper, Pro Armour Footpeg Extensions, Renthal Bars, Yoshimura Full Exhaust, 450R Carb w/ZR's Adaptor, K&N w/o Lid, Powroll 440 Stroker w/stg 3 HC, P&P Head, HD rod, GT Thunder HD Studs, and Timing chain, AMR Monster Coil, 4 Degree Advance Key and a 12oz lighter flywheel. Now at 12.5:1 Compression!
450R Front brake upgrade using 06-up Spindles and 07' hubs.
450R Rear Brake Upgrade
Kenda Knarly XC Radial 21x11x9 Rear Tires
Shorai Lithium Iron Battery
Running E85--Jetting: 70 pilot, 240 main, HRC needle on bottom notch and shimmed.