Introduction of 1024QAM
One subcarrier now carries up to 10 bit information in 11ax. With 1024 QAM, 11ax has higher data rate as MCS10 (1024QAM, 3/4 coding rate) and MCS11 (1024QAM, 5/6 coding rate). LDPC is mandatory and BCC cannot be used in MCS10 and MCS11 for coding scheme. For more about modulation, find Modulation & Constellation chapter.
Comparison on MCS9 between 11ac and 11ax
Logically, with 4 more subcarriers in 4 times longer symbol, 11ax OFDM makes the same data rate of 11ac OFDM. With the same modulation and coding rate, and with the longest GI (Long GI for 11ac and 3.2usec GI for 11ax), VHT80 and HE80 data rate are 390Mbps and 408.33Mbps each. The reason they have a small amount of difference is the number of subcarrier utilized. 234 data subcarriers in 11ac and 980 data subcarriers in 11ax in case of 80MHz BW. 980 is slightly over 4 times of 234. (234 x 4 = 936 < 980)
For reference, the numbers of data subcarriers in HE20, 40 and 80 are 234, 468 and 980.
Comparison on highest data rate between 11ac and 11ax
Along with Normal GI, 2 bit more information on each subcarrier and 4 times more subcarrier in one symbol in 11ax masks a big difference (about 30%) between 11ac and 11ax at highest data rate.
Data rate for HE20/40/80/160 of SU and full RU
Data rate for 26, 52, 106 tone RU
HE data rate with MIMO, DCM and GI
Data rate with DCM is half of data rate without DCM. With normal GI (0.8 usec), data rate goes up by 15% compared with Quadrature GI (3.2 usec)