The minimal supersymmetric SU(5) grand unified theory (GUT) requires equality of down-quark and lepton Yukawa matrices at the GUT scale. Fulfilling this condition for the first two generations in the MSSM can be achieved by an appropriate choice of soft supersymmetry-breaking parameters. Either non-universal A-terms or flavour-off-diagonal squark mass matrices can generate sizeable threshold corrections to the small Yukawa couplings at the SUSY decoupling scale. We investigate whether such corrections can be large enough without violating constraints from flavour observables and electroweak symmetry breaking.