A288a: Neither, it's at 15 HD (11 +4).
A288b: By RAW it would not qualify for the full increase, but IMO clay golems are particularly nasty sons of bitches with their SR vs. Healing curse (an Ex ability for you polymorphers out there), the DC of which appears to be Strength-based with a +4 racial bonus.
For example, a 22 HD Clay Golem advances to Huge size and gains 3 stat bumps from increased HD. They also gain a mere +3 adjustment to their CR, bringing it to 13.
Advanced Clay Golem -- CR 13
Always N Huge Construct
Init -1; Senses Darkvision 60', low-light vision; Spot +0, Listen +0
Languages --
----------------------------------------
AC 24 (-2 Size, -1 Dex, +17 Natural)
Touch 7, Flat-footed 24
HP 160 (22 HD)
DR 10/adamanite and bludgeoning
Immune Construct Immunities
Fort +7, Ref +6, Will +7
----------------------------------------
Speed 20'
Melee 2 slams +26 melee (4d8+12 plus Cursed Wound)
Space 15', Reach 15'
Base Attack +16, Grapple +36
Attack Options Cursed Wound DC 33
----------------------------------------
Abilities: Str 35, Dex 8, Con --, Int --, Wis 11, Cha 1
SA: Berserk, Cursed Wound
SQ: Construct Traits, DR 10/adamanite and bludgeoning, Darkvision 60', haste, immunity to magic, low-light vision
Personally, I think that's a really nasty CR 13 encounter. You need serious magic to reverse Cursed Wound (I believe the Break Enchantment DC is 33 as well), and even the stuff that *can* break it can't be cast in combat. If the group's beatstick can't take it out in one shot, someone's going to get some serious pain laid down on them.
In other words, just don't follow the guidelines listed in the MM/DMG/wherever. Best way to deal with XP in D&D is to tailor the encounters to the group and then award an arbitrary amount of XP based on their advancement of the campaign.
