No, PGA gain and update rate are indepent settings.
Both ADC and PGA incorporate noise sources, by varying the update rate and gain, the output noise level will be changed. The relation between update rate and noise is primarly a matter of different noise bandwidth, in addition the specific noise-shaping characteristic of delta-sigma converters involves an overproportional increase above a certain sampling rate.
Discussing the noise proporties of a particular ADC isn't very meaningful without considering the signal properties, however.