I can personally tell you that moving a 8-D around is a PITA. I'm a big boy but getting old and my shoulders kill me for a week after moving it.Otherwise it no different than the grp 27 except a even shallower drop in voltage over night before the system starts charging again.
Actually come to think of it, the last time I had just the 8-D on I had been running it down to give the solar something real to charge. I tested the 8-D before bed and it read 12.3v. It ran the furnace that night and when I tested it the next morning before sun up, it still read 12.3v.
When it's time to replace these batteries I'm going with a pair of 6v and be done with it.