I do believe this has been finalized by the 'proper authorities' now... Link and quote:
http://paizo.com/paizo/faq/v5748nruor1g1#v5748eaic9p07
Alchemist: Is the Strength bonus for the rage mutagen ability of the ragechemist archetype (page 25) in addition to the normal bonus for a Strength mutagen?
No, the +6 replaces the normal +4 Strength bonus of the alchemist’s Strength mutagen. This will be updated in a future printing of Ultimate Combat as follows:
Page 25—In the Ragechemist archetype, in the Rage Mutagen class feature, change the first sentence to read as follows:
"At 2nd level, whenever a ragechemist creates a mutagen that improves his Strength, that mutagen’s bonus to Strength increases by +2 and penalizes the alchemist’s Intelligence score."
—Sean K Reynolds, 02/21/12