What is the actual capacity & charging current of a given deep cycle lead acid battery?
doesn't appear to be significantly different between 20% and 80% DoD. 


It appears to me if the DoD is 20% (80% SoC) you will get almost 5,000 cycles.  If discharged to a depth of 80% the number of cycles will only be about 1,000, or 80% less life.


  Is there in fact a possibility to use a higher discharge current for
  gel/AGM vs lead acid?


Gel and AGM are Lead Acid. You may mean Flooded Lead Acid.  Discharge current is different for each type of Lead Acid.  An AGM can be charged up to 5 times faster than a flooded or gel. Be aware that AGM does not like heat and a faster charge rate can increase the temperature.

For general info see: Battery University Learn  Section: Battery Types
BU-201: How does the Lead Acid Battery Work?
BU-201a: Absorbent Glass Mat (AGM)
BU-201b: Gel Lead Acid Battery 


  in just over 4.5 hours?


No, typical flooded lead acid (e.g.SPRE 12 225) charge time is 12-16 hr. See also  Charging Lead Acid.  Your 20% to 80% is ambiguous.  Do these percentages refer to SoC (capacity) or DoD (discharge depth)?  If the battery is discharged (DoD) 20% (80% capacity remaining SoC=80%) then charge time could reasonably be 4.5 hr, but I did not see that information in the linked document.