This battery is a (sealed lead-acid) "starter" type battery, i.e., designed to provide a large current for a short period (rated 170 amps CCA). Perhaps there is something about a slow charge that may change the shape of the lead compounds on the electrodes in a way that will reduce that cranking ability? I don't know. But for the typical battery used in a non-motorized glider I still recommend a slow charge, generally no faster than the discharge.

A 0.4C charger is not terribly fast, it would take 2.5 hours for a full charge of an empty battery if the current was constant, which as you said it isn't, so more like 4+ hours. Of course you shouldn't ever fully discharge a lead-acid battery, thus it would rarely need a full charge cycle.

A lithium battery can be "fully" discharged (to the point where its internal controller says "no more") with less effect on its longevity. That means that, for example, a 10AH lithium battery will provide as much practical usage time as a 15 or 20 AH SLA. This doubles again the weight advantage. But still at 6x the price. For that reason I'm still using SLA. It helps that the modest gizmos in my glider allow me to fly 6 hours on a 5AH SLA battery. (I use the 12AH battery on strong days, that's my "ballast".) If the LiFePO4 price comes down another 2x, and/or I'm convinced of a long-enough life in years, I might spring for it.