writes:

WAAS is a subsystem of GPS that transmits localized GPS correction
information for the US via a pair of geosynchronous satellites.

No, WAAS is an independent system that relies in part on GPS. There are many
such systems (indeed, anyone can set up such a system).

With WAAS corrections, the maximum lateral and vertical error is 7.6 m
under all cicumstances, makeing it more than reliable enough to make
vertically-guided landing approaches in IFR.

With augmentation you can theoretically get any accuracy you want.