GPS L5C Differential? Max Update Rate(bits/s|Hz)?
The L1/L5 frequency pair can be used to compensate for ionospheric delay much in the same way as L1/L2. The wider spacing of L1/L5 as compared to L1/L2 will not enhance accuracy significantly.

In theory, if using all of L1/L2/L5, second order ionospheric effects can be eliminated (see here). We will see if this gives superior correction when manufacturers adopt this (or a similar) method (I'm sceptic).

The quadrature part of L5 is used as a pilot and does not carry data modulation. A convolutional coder expands the net data rate of 50bit/s into 100 symbols/s (to shape the power spectral density of the signal in space). These are phase shift modulated onto the inphase part of L5. Net rate is 50bit/s.

WAAS and EGNOS send 250 net bits/s in 500 symbols/sec. The relevant documentation (MOPS DO-229) is behind paywalls, I cannot link it here.

BTW: The term "differential" does not apply here, it is used when signals from at least two different antennas contribute to the solution (either independent receivers or antenna arrays).