For the real story on how VRLA (Valve Regulated Lead Acid = AGM and Gel) batteries discharge, should be discharged, charge, and should be charged ... this link is my go-to document. This technical bulletin is far more comprehensive and educational than any other AGM battery information that I've come across.
Note from studying this document, that high charge rates (as high as C/1) are possible - but control of battery temperature must be maintained at high charge current rates. However, high powered battery chargers with automatic control of their output via an external probe that monitors battery temperature are hard to find.:
http://www.cdtechno.com/pdf/ref/41_2128_0212.pdf