Re: Brokenness
From Forgotten Realms Campaign Setting (3 abuses found, all others have a BAB requirement equal to the level necessary for the skills, or the skills are lower, or corresponding casting ability).
-Divine Seeker: primary entrance skill - Hide 10 ranks (7th level). Other prereqs include Move Silently 8 ranks, Spot 5 ranks, Knowledge (Religion) 3 ranks, Patron.
Rog5 can get in as opposed to Rog7 at the expense of 1 feat.
-Guild Thief: primary entrance skill - Hide 8 ranks (5th level). Other prereqs include Gather Info, Intimidate, Move Silently each 3 ranks, Membership.
Rog3 can get in as opposed to Rog5 at the expense of 1 feat. Big Abuses, Reputation can boost Leadership by 1 at 6th level, leading to an inflated cohort.
-Harper Scout: primary entrance skill - Diplomacy 8 ranks (5th level). Other prereqs include Perform 5 ranks, Knowledge (Local) 4 ranks, Sense Motive and Wilderness Lore 2 ranks each, Alertness, Iron Will, Sponsorship.
Rog3 or Brd3 (Human Only) can abuse this. Earlier spell access and possible Favored Enemy abuse.
From Forgotten Realms Magic of Faerun (2 abuses found, all others have 1 high skill requirement corresponding to a high level casting ability (usually 2-4th level spells)).
-Mystic Wanderer: primary entrance skill - Diplomacy 8 ranks (5th level). Other pre-reqs include Alchemy, Knowledge (nature), Perform, and Profession (Herbalist) 3 ranks each, Iron Will, and the ability to cast 2nd level divine spells.
Drd3
1 feat - Iron Will
1 feat - Skill Focus (Diplomacy +2 ranks)
With proper skill selection (Alchemy and Perform are cross class):
Total Skill Points (no Int bonus): 24
Diplomacy 6
Knowledge (Nature) 3
Profession Herbalist 3
Alchemy 3
Perform 3
No points left over. Any level 3 druid can get into this prc by level 4. Big abuses, Timeless Body by 13th level.
-Spellfire Channeler: primary entrance skill Concentration 8 ranks (5th level). Other prereqs include Knowledge (arcana) and Spellcraft 2 ranks each, Endurance, and Spellfire Wielder.
Wiz3 or Sor1/Mnk2
1 feat - Spellfire Wielder
1 feat - Endurance
Bonus Human Feat - Skill Focus (Concentration +2 ranks)
With proper skill selection (no cross-class skills), you can easily get in 2 levels early. Big Abuses, Spellfire in general, esp healing, flight, and maelstrom of fire 2 levels early.
From Forgotten Realms Silver Marches (0 abuses found, all prc have a BAB requirement equal to the level necessary for the skills, or the skills are lower, or corresponding casting ability is necessary).
From Forgotten Realms Faiths and Pantheons (3 possible abuses found, all other have a BAB requirement equal to the level necessary for the skills, or the skills are lower, or corresponding casting ability is necessary).
Horned Harbringer and Ocular Adept rely on 1 skill and skill focus can get them in 2 levels early. Possible abuses: Horned Harbringer - None, requires DM permission as you much touch an artifact. Ocular Adept - None, requires DM permission as you much swear allegiance to beholders and they must accept you.
-Techsmith, can get in as early as 4th, so biggest abuse lies with Human Sorceror (who has 3 feats, 2 required for class and 1 for skill focus by 3rd level). Still somewhat adjucated by DM as you must visit High Holy Crafthouse of Inspiration on Lantan, not easy to do as a low level character if your DM doesnt want you to.
From Forgotten Realms Races of Faerun (2 abuses found)
-Great Rift Skyguard, ride primary skill, allows access to class 2 level early (3rd instead of 5th). Possible Abuse, Hippogriff Steed by 5th level.
-Warrior Skald, perform primary skill, allows acces to class 2 levels early (4th instead of 6th). Possible Abuse, not really, just super bardish class.
Despite my extreme skepticism, I could not find any issues Realms-wise (beyond those outlined above). Most prcs seem designed with 2 prerequisites that limit the level, IE- High Skill ranks in 1 skill AND BAB or High Skill ranks in 1 skill AND Spellcasting Requirements. I would still go through my splatbooks and check those for possible abuses before I let a feat like that in my campaign, also make note that any abuse of Perform to enter a prc doubles its utility for a bard as it grants earlier access to songs.
Technik
PS- Not renaming the feat to someting besides "Skill Focus" opens another can of worms in and of itself. If you can use the feat to gain 2 ranks in a required skill for a prc, plus bypass the requirement of needing skill focus in the first place, you have gained a lot of ground. This is handy for Loremaster or Archmage, both of whom have a Skill Focus requirement AND a High Skill requirement of the same skill focus is need in. Have to point out that you would need to take Skill Focus twice in either of those cases, 2 feats, 2 levels early, too much of a hit for Archmage, but possibly worth it for Loremaster.