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Subsonic: I've done such calculations in the past and at first I was a bit puzzled too. I can give you a hint that your model is not that accurate. One issue is that you're trying to calculate the moment of inertia from mass and disk radius, but you don't know how the mass is distributed on the pulley. The OEM pulley is more heavy on the outside part of the disk and the mass is not distributed evenly like your model. This means that the OEM's pulley inertia is much bigger than your number and depending the manufacturer the lightweight pulley's inertia can be much smaller than your number. Overall, it is a complicated model with too many parameters and it is the reason to insist that the only way to get a close to reality result is to make your own acceleration measurements or just stay with dyno numbers.
I placed the following dyno many times in the past and I'll place it once more. And believe me the particular manufacturer is mainly an engine builder and they would never lie with numbers (*).
(*) I asked them once about gains on an exhaust and they were sincere to say that they didn't have any measurements on the particular engine. They don't care to sell by just posting non-sense or telling lies.