One more thing to consider.  If there's any possibility of shutting down the heat for more than a few days for any reason (vacation, health, weather related issues, etc.)  then foam under the entire slab will allow it to be warmed up quicker when the reheating begins.  Without foam, it takes just as many days to warm it up as it does to cool it down (if frost gets into the dirt under the slab, it might take 7-10 days or longer) -- with foam, it takes about 4-8 hours depending on actual temps and amount of mass (slab, equipment, tools, etc) to be warmed in shop.

IMO, the only reason to leave out the foam from under the middle of the floor is if the heat is going to be constant all winter long, and then I still have some unresolved feelings about having to warm the dirt under the floor to the same temp as the slab. 

The only time it's 'pays' to have the heat sink is if the power is off more than a couple of hours but less than 24 hours, when the power comes back on the shop is very close to the same temp.  If this only happens once a winter, is it worth it to heat the dirt all winter for that one day? 

Around 'here', we've been very fortunate to have only lost power for more than a couple of hours, maybe twice in the last 15 years.  Your mileage may vary.

something to consider