"Is it reasonable that, during the implementation period, when millions of business plans and careers based on the 'old system' are severely disrupted by such an extreme and radical change, that there'd be any economic expansion whatsoever?"
You miss two things, WG.
(1) Because of pricing shifts, US produced goods will experience INCREASED demand, both here in the US, and in many markets around the world. That type of "disruption" would be most welcome by our agricultural and manufacturing sectors.
(2) Most of the other "disruptions" would be positive in nature as it pertains to US business. Do you honestly believe that ANY business will be saying "Bring back the corporate income tax and payroll taxes, we just can't survive without them!"?
So to answer your question, yes, absolutely I believe that US businesses would start to expand immediately (and probably a few weeks ahead of actual implementation) to take advantage of the greatly expanded opportunity to market US produced goods.
What "US produced goods"????
When I go to the store, I don't see any "US produced goods".
You must live in some kind of fantasyworld.
Because of "pricing shifts", and the dollar declines to purchasing power parity with the peso, rupee and yuan, The American standard of living will also decline to the squalid poverty levels of third world nations like Mexico, India and China.
THAT is the outcome of your pathetic "tax reform".