Further to my post above, I had the chance to browse the manufacturer's website. They have excellent detailed PDF information on their red-top batteries.
They are, in fact, AGMs, but they've been "optimized" for both standby service
(as in telephones, UPS, etc.) and for deep-cycle service
(as in wheelchairs, golf carts, boats, etc.). They've done this, apparently, by changing the grid material and other internal elements.
They mention that no AGM
battery can match gelled batteries for deep-cycle service, but say that their red-top AGMs come close:
So, just be careful to charge at the recommended 2.35VDC per cell (14.1VDC for a 12V battery), and have fun. They should last quite a long time :-)