perhaps you've heard about rogues using guns to deliver sneak attacks. It's not bad, but as they can't focus well in rapid reloading, and flat footed is quite rare to deliver ranged sneak attack, it's not a well optimized build.
As you seem to houserule allow it just for real gunslingers, you only have to worry about multiclassing.
Else there might be some things that give an extra 1d6 damage, but I don't know of any real sneak attack progression.
First off, the BAB is only +6, not +6/+1. The +6/+1 is a secondary attack granted by having a BAB of higher than +5. A kind of weird technical difference, but it helps when figuring things out, and here's why:
the Secondly>>, the penalty applies to the first attack roll. In effect, you get a first attack at +6, a second (off hand) attack at +6, a secondary attack at +1 and a secondary (off hand) attack at +1. This effectively makes his atack bonus (not BAB) +6/+6/+1/+1
In addition there will be penalties for two weapons (there's a chart somewhere), lessened by two weapon fighting, and possibly by being light weapons. Off the top of my head I can't remember whether pistols are considered light weapons. For the purposes of the above calculations I didn't double check the chart about two weapon fighting, and presumed the pistols are light weapons (which I think effectively neutralizes the penalty). If pistols aren't considered light, then the attack sequence would look more like +4/+4/-1/-1, so he still gets four attacks, but with less chance to hit per attack.
And on a final note, without Improved Two Weapon Fighting, his attack sequence would be +6/+6/+1 (or +4/+4/-1, with the whole light weapon thing...)
Since you count pistols as light weapons, then yes, the additional -2 penalty is dropped.
And yes, the secondary attack for having a higher than +5 BAB already includes the minus five penalty. At eleventh level, where the gunslinger's BAB is +11, the attack sequence (without off hand attacks cluttering it up) would be this: +11/+6/+1 That is +11 for BAB, minus five to get a secondary attack at +6, and minus another five for a tertiary attack at +1.
By taking an off hand attack, you get one extra attack at your BAB (but all attacks get penalized. These penalties are reduced in two ways. First is the two weapon fighting feat, and the second is by having a light weapon. At every level where a new attack is gained (sixth, eleventh, sixteenth), there is a new feat that allows you to make an extra attack, and is listed with a corresponding penalty. -5, -10, -15. Perhaps it is better to ignore the penalty and think of the feat as giving the character the bonus attack, but with another hand.
So with the primary hand, you get +6/+1 @ sixth level
With Improved Two weapon you get the same for the off hand (re: +6/+1 right, +6/+1 left)
Without Improved Two Weapon, you only get One extra attack with the off hand
At eleventh you have +11/+6/+1 with your primary hand
Without Improved, you get +11 with the off hand.
Improved gives you +11/+6
Greater two weapon fighting gives you yet another attack, basically equal to the lowest in your unmodified chain (re: +11/+6/+1)
I sort of hate to complicate things here, but I'm going to go ahead and point out that +6/+1 at sixth level implies taking the full round to attack. It works the same as melee, so if you want the extra attacks (the secondary attacks), all the character can do that round is attack.
I forget whether a five foot step can be made, but I have a rules lawyer to answer that for me in the game ;)
Hope I didn't confuse you more, and also that maybe I helped you understand a little bit. I'm not always the best at explaining math...
Just double checked my book. I made a small error. The core rule book does not have a two weapon feat at level sixteen to give you that attack. I could have sworn I saw it somewhere, probably Ultimate Combat, but might have just made it up myself. Then again, who really needs to make 8 attacks a round with a fifteen point difference between the highest and lowest?
Gunslingers cannot sneak attack unless they multiclass; however, the pisolero gets the Up Close and Deadly deed at 1st level, which is similar to Sneak Attack (spend a grit point to add precision damage to one attack instead of when attacking a flat-footed opponent).
Double-barreled pistols, alchemical cartridges, weapon cords, Improved Two-Weapon Fighting, Point Blank Shot, Rapid Reload (double-barreled pistol), Rapid Shot, and Two-Weapon Fighting can give you five attacks per round: fire, fire (Rapid Shot), drop off-hand pistol (free action), reload (free action with Rapid Reload and alchemical cartridges), reload (free action), fire, reload (free action) with primary pistol; retrieve off-hand pistol (swift action with weapon cord), fire, fire, drop primary pistol (free action), reload (free action), reload (free action) with off-hand pistol; rinse-repeat with the off-hand pistol as the primary pistol next round (and the "off-hand pistol" already dropped). Granted, all of your attack rolls are at -6 (-4 for two-weapon fighting with two one-handed weapons, -2 for Rapid Shot; +1/+1/-4//+1/-4 at 7th level, not counting Dex and other bonuses, when you can take Improved Two-Weapon Fighting), but you attack touch AC in the first range increment (20 ft; 30 ft if under the effect of longshot; a level dip in alchemist, inquisitor, magus, sorcerer*, or wizard* might be worth considering).
*- Take the Magical Lineage (longshot) trait and Still Spell; truestrike has no somatic component and can be cast in armor without a chance of failure. A wizard dip may wish to go spellslinger and attune both double barreled pistols.