If an 8 second Penance is OP (not saying it is or isn't), then remove the Glyph of Penance, and problem solved.
Instead, Blizzard now FORCES Disc Priests to use the Glyph of Penance, since no Priest would want to trade an 8 second cooldown (live) for a 12 second cooldown (3.2).
Had Blizzard simply changed/removed the Glyph of Penance, Priests would be left with the same cooldown of 10 seconds, and it would free up a glyph slot that could be used for other things. But instead, Blizzard took the clumsy and lazy path, and has essentially removed 1 glyph slot from every Disc Priests spellbook.
GFG