Yeah, rules seem pretty clear on this one. Most people trying to pull shenanigans with this want to add bonuses for the substituted skill on top of the perform bonus. The DM would be right to shut that down. But the RAW is “When you make a skill check, you roll 1d20 and then add your ranks and the appropriate ability score modifier to the result of this check. If the skill you're using is a class skill (and you have invested ranks into that skill), you gain a +3 bonus on the check... Skills can be further modified by a wide variety of sources—by your race, by a class ability, by equipment, by spell effects or magic items, and so on." A DM can of course make house rules, but there ought to be a reason, and it shouldn’t be a surprise to a player on a major class ability.