You don't need to account for the lack or the formation of the octet in this compound: when SiF4 is formed, even though the Lewis acidity is relatively low( due to retrodative 2p :rarrow:3d bonds), fluoride is a hard nucleophile and manages to overcome the octet stabilization and attacks with his lone pair via LUMO of silicium, which is one 3d. And 3d orbitals aren't degenerated anymore, can you tell why? ( the same reason as at Crystal field theory).
Anyway, this compund shows best the way the energies compense and overcome metastable states, to reach a maximum stability.
P.S: can you tell why SiCl62- doesn't form by dissolving, say, H4SiO4 in concentrated HCl?