As Far as im aware the spring sets are the same for T5, T5.1 T6 in respect of size/diamentions.
But the load ratings should be observed so T6 T26 > T5.1 T26 as the spring differ in stiffness.
Though i did fit T32 rear springs on a T30 T5.1 due to weight issues causing the rear of the van to sag with the stock T30 springs, T32 rear springs solved the problem.
This was a VW Main dealer approved swap.