Hi,
More bits doesn't mean better performance.
I've seen a lot of 24 bits ADCs that don't have better performance than 16 but ADCs.
And honestly I've not seen an ADC with true 24 bit performance.
My recommendation: forget about 28 bits.
The other point is, that even if you have an ADC with true 24 bit performance..you need to design the surrounding circuit to keep this performance.
I'm doing high precision electronics for decades now, but I don't think I'm able to design a true 24 bit performance system.
For a 3V range a 24bit resolution means about 180nV/LSB.
Thermocouple effects and even mechanical stress will make 24bits precision impossible.
If I want to know this ... I'd use selection guides and chart from manufacturers and distributors.
Klaus