I don't think its just VVEL, exhaust cam making 400 whp. Seems to be a combination of other mods and tuning on top of it.
their stock cars are 340 HP at crankcase
305 whp on Bosch dyno
350 whp with exhaust mods
360 whp tuning for above including AFR, IGN timing
378 whp probably with exhaust camshafts
once you have control over VVEL at higher deg and lift for good range of powerband, maybe this can help in tuning the Ignition timing more, netting CLOSE to 400 whp which is 22 whp more than the last step.
Maybe I'm wrong