Given the info we've seen about 21CN ADSL, those 3 options (Standard, Stable, Super-Stable) only affect the thresholds that DLM uses to decide when (or whether) to change up or down to a "better" or "worse" profile, and not what changes the new profile will choose to make... and it is the profile that determines whether you have any speed restrictions and/or any interleaving added.
For example, the good/bad MTBE thresholds were shown as 60 and 600 for "standard", 600 and 6,000 for stable, then 6,000 and 60,000 for super-stable.
While that was about ADSL, the DLM for FTTC is likely to work on similar principles (particularly as it has the same classifications for the setting).
For FTTC, the ISP doesn't have any controls to directly influence the use of interleaving, nor any way to force DLM to choose profiles differently (eg change speed rather than interleaving, or vice-versa).
* - The word 'profile' is used here for DLM profiles, not the speed-based IP profiles