How much energy in amp hours or wattage does one loose in (this example) a 17ah SLA at 13.8v compared to 14.2v or 14.6v ? , it is just a thought I dwelled on and not really critical for here .
The short answer: In terms of usable capacity, almost none. Any voltage over and above that required to keep the battery floated is simply wasted in excess gassing / heat.
(edit: Personally, I wouldn't float a SLA meant for stationary use at any more than 2.27V/cell @ 25 degrees C e.g. 13.6V for a nominally 12V battery. But that's an opinion based on many years of looking after the damned things in telco applications, and will differ slightly with the exact materials & construction used.)