I have no doubt that some groups can put together very poor simulation models - I've seen government created software, and I have little faith in it. But I expect that there are professional groups that can put models together that are back tested and verified to be good at predicting the outcome of an econometric change such as this, and at low cost compared with the potential cost of failure and disruption in this radical change in taxation method.
You're selling this as a "Fair Tax", I assume that you mean that everybody currently being taxed will not see a significant change in their tax burden. I do not believe that. I would like to see some verifiable scientific proof of that claim, not a philosophical blow job.
Actually, I think the current tax system is icredibly unfair. So having everybody come out the same is not a priority of mine.
I like the FairTax for the philosophical reasons. Honesty, fairness, and removing the lobbyist factor from the revenue process.