Max redo speed on replica side in Availability Group
"Heard about"? From where? As far as I know this is dependent on local factors such as raw disk subsystem capability, controller capacity, concurrency, type of data being logged, etc. etc. 

Redoing large index maintenance operations on AGs is definitely a known issue. This post certainly admits as much and suggests ways to mitigate the issues:


Recommendations for Index Maintenance with AlwaysOn Availability Groups


But I don't think you can expect any predictable x MB/sec never mind a guarantee, you will have to observe what your system is capable of, and troubleshoot issues when it falls below your threshold range.