The telemetry functions for the 8 & 18 are basically identical.
They both log the data to the SD card(with current Airware).
The TM1100 provides basic telemetry but lacks the X-Bus port that the TM1000 has to add optional sensors.
Im not sure what leads you to think the DX8 does not log the data.Originally this was not a feature but has been added recently.Could be you got outdated info.
The only possible significant difference between the 8 and 18 is the dual antenna diversity in the 18 that does in my experience make the returning telemetry signal(and RF in general) more reliable.Beyond that the menus and options are functionally the same with one exception.
A menu for JetCat ECU data is being added from what I understand.IIRC it is only going to be available in the 18 at least for now.
It seems to make sense that they would add new features to the upper end radios first and then possibly some may trickle down based on user demand.We have seen how our radios are constantly evolving.I wouldnt expect that to change.