Been running a Sibler stage 2 (with A2D clutching) for about a month and a half now and I have NO reason to doubt it's claimed +20 HP. Off idle response is indiscernible from stock and the map seems to run just as clean. Can't speak for Stage 3 or 4 as I haven't ran them yet. Haven't felt the...
It's not an apples to apples comparison since the Vipers are geared lower than the Nytros, My 240 OFT kit also needs heavier weights on my viper relative to the same setup in the Nytro.