Has little to do with a "buffer", which might represent 5% of the battery capacity. The Taycan has 80KwH usable and thus charging at 270KW is 3.4C rating. C rating is charge rate / battery capacity. At 200KW charge rate the Rivian battery is at 1.5 C, at 300 KW it will be at 2.2C, still less than the Taycan. Taycan starts to taper at 25% SoC because of that high charge rate.Well that 100% depends if it has a buffer like Taycan or not, and the charging curve. If R1T is 300 miles at using 100% of battery then you're wrong, if its 90% of battery, it's pretty close but again would depend on the curve(at 900v)
A Max Pack at 300KW would be charging at 1.7C.
As a reference point the Audi E-Tron charges at 1.8C all the way from 0% to 70% before it tapers.