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super flat floors

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Good morning all:
Does anybody out there have experience with super flat slab on grade floors. In our case we have an area of about 200' x 300' with racking height of about 32'. We have spec'd a concrete strength of 25MPa (28 days) with steel fibres, 1 1/2" aggregate and a w/c of 0.55. We are not experts in this fields and as a result are telling the people building it the specs we require and are basically putting the ball in their court. Initially we spec'd a 30MPa concrete with a 0.45+- 0.02 w/c ratio but the concrete supplier said that this mix would crack too much. He tells us that with the cement that they use and the low w/c ratio the strengths would be in the 40+ MPa range and would cause a lot of shrinkage. We are obviously trying to reduce shrinkage and thought that the low w/c ratio along with a requirement for the design slump to be 1 1/2" with super-p being added to increase slump to the 5" range would produce the least shrinkage. We are being told that a w/c ratio of 0.55 or even 0.6 is preferred by the concrete supplier.  The concrete supplier says he takes no responsibility for the mix and has admitted that we can't get guidance from others in different locales because the ingredients are of differing qualities and chemistry than what we deal with in our area.  I have an article from concrete international that recommended 4000 psi with the 0.45+- 0.02 w/c ratio. The concrete supplier has said that he has heard others in the US swear by this mix however he said that if he produces this concrete with his ingredients the strength is far too high and as a result there is more shrinkage cracking.  There does some to be some consensus in what he is saying based upon projects that have been done in our area. Water reducers which I previously thought would reduce shrinkage are actually increasing shrinkage??(according to ACI 360R-06) Why use them at all then?
What are the key elements that you would use to spec the concrete. We need a final strength in the 4000 psi (28 MPa) range. There is only PC type 10 and blast furnace slag as the cementitious components, the aggregate is limestone. There are no post-tensioned slabs done in our area. The floor people want to let the floor crack and will fill the joints as long in the future as practically possible..perhaps 6 months..with epoxy.
any thoughts