Number of Turns for Ferrite Transformer Considering the Design of Laminated Transformer

I have a toroidal transformer with 18 / 18 turns in primary and secondary windings of hollow copper tube , the primary and secondary voltages being 600/600 Volts MF @ 10Khz, And Crosssectional area of M4 laminated core is 7200 Sq.mm, 300 Kva.

Now if the same transformer is to be converted to Pancake type of windings , in a manner such that it consists of 4 pancake coils with the same cross sectional area of conductor used above with 9 nos of turns in each layer of winding, ( 2 primary windings in series placed externally and 2 secondary windings in series placed internally like a sandwich ), is there a relation for calculation of ferrite crosssectional area if ferrites of Bs 390 mT @ 100deg,C temperature ( H=1000 A/m for Freq= 10 KHz). The Cross sectional area arrived by calculation is 1900 Sq.mm for above ferrite, Please guide me. If more than required cross sectional area of ferrite is used , is there any loss of efficiency of transformer.The tubings are cooled internally by pressurised water.