Sepulchrave-> "Maybe I should phrase the question differently. If the elite array (25-point) is worth +1CR, how much is a 54-point build (16,16,16,15,15,15) worth?"
I sometimes I use the Elite Array +2 (17/16/15/14/12/10) for noble or fated NPCs in my campaign and while it makes them more powerful among those of their CR, that by virtue in itself didn't really warrant bumping them up a whole CR even, they'd just be a "tough CR 12" or whatever, like Aasimars are technically +1 ECL though very low on that end. For 16, 16, 16, 15, 15, 15 I'd probably add +1 CR to most monsters, (rewarding the players extra gold for that CR) - But then compare how much these monsters are actually challenging your players, if +2-3 AC/Saves/DCs/extra HP/Attacks/Damage is fairly powerful, bump it up to +2 CR. If it's only mildly useful, +1 seems safe. Heaven knows Epic games are very hard to adjudicate CR wise anyways (to me it feels like a Paper/Rock/Scissors game mostly), and a lot of the monster CRs in the ELH seem much underpriced themselves.
If it helps recall in a WotC supplement an item that gave +6 enhancement bonus to all stats, but was priced about as much as 4x one of those items (~144,000 gp). From a FAQ when someone asked why it was so underpriced and the answer was that very few classes and monsters benefit from all six stats so the remaining ones were given at a discount, which was kind of silly in retrospect since it still only took up one item slot, but that was their logic. Sorry I can't get you more detailed information, it could've been the Headband of Perfection from Sword & Fist but IIRC that only gave +6 to 3 stats.
Rackhir-> Monsters getting the Elite array for classes for free makes sense to me because a lot of times, they could get double or triple the HD if they advanced by their monster type and often the lowest level class abilities, even in associated levels, don't add to their effectiveness much (Sneak Attack +1d6 and Evasion for +2 CR instead of 6 HD?) or would help a significant amount if they had some boosted up scores (Spells, Stunning Fist, Divine Grace, etc..) The Elite Array may be a bit unfair to give for free for just one level, but once you get up to 3+ it seems to make up for the lack of direct power they're getting.