Okay, starting from the top:
Bloodweaver1 said:
Say a warforged is a first level monk
Warforged Monk (1): Unarmed Strike 1d6, Slam 1d4. If he wants to use the slam and the unarmed strike in the same round, he's got to take the normal penalties for mixing natural and manufacured weapons (-5 to hit, -2 with multiattack)
and has two battle fists (one for each hand), what is its unarmed damage score?
First off, I'd say (quite firmly) that the benefit of two battlefists do not stack. That is to say he does not get
2 size adjustments, etc. Just one. If, for the sake of appearance, you want your battlefists to be a pair of battlefists that's perfectly cool, but you don't gain the effect twice. Anyway, that said.
Warforged Monk (1) with Battlefists: Unarmed Strike 1d8, Slam 1d8. Both unarmed attacks and slams now have a +1 enhancement bonus. The warforged is now using the large creature table for monk unarmed damage.
Same warforged at thrid level likes the damage he is doing and picks Supreme Unarmed Strike for a bit more pain. What is its unarmed damage score?
You need to be a 4th level monk in order to take Supreme Unarmed Strike, because of the BAB requirement (+3). For the sake of simplicity, we'll say that the DM is a nice guy and lets you take Supreme Unarmed Strike as your lvl 6 monk bonus feat, that way it doesn't mess with the next point, below.
Warforged Monk (6) with Battlefists and Supreme Unarmed Strike: Unarmed Strike 2d8, Slam 1d8. Both have a +1 enhancement bonus. The warforged is effectivly attacking with their unarmed strike as if they were a 10th level large monk.
Adventureing is going well. After barely defeating a BBEG it reaches sixth level and comes to realize it needs to do more damage. So it picks up Improved Natural Attack. What is its unarmed damage score?
Warforged Monk (6) with Battlefists, Supreme Unarmed Strike, and Improved Natural Attack (Unarmed Strike): Unarmed Strike 3d8, Slam 1d8. Both have a +1 enhancement bonus. The warforged is now effectivly attacking with their unarmed strike as if they were a 10th level huge monk.
After rumaging through the BBEG's treasure chest, it finds a 'Monk's Belt' and puts it on. Finally, what is its umarmed damage score now?
Warforged Monk (6) with Battlefists, Supreme Unarmed Strike, Improved Natural Attack (Unarmed Strike), and Monk's Belt: Unarmed Strike 4d6, Slam 1d8. Both have a +1 enhancement bonus. The warforged is now effectivly attacking with their unarmed strike as if they were a 14th level huge monk.
So ultimately what stacks, what doesn't, and how much damage could a sixth level warforged monk do with Flurry of Blows.
What stacks: Just about everything, other than the dual battlefists (as they're the same item). An argument could be made for the battlefists not stacking with INA because they're both effective size increases, but neither have any notes to the contrary. Of the set, that's the most gray area. Multiple effective size increases.
As for how much damage the monk could put out? A lot. Assuming they hit, anyway. Assuming they flurry (a better option than including the slam, at this point) and both attacks hit, you'd be looking at 10-50 damage not counting possible strength modifier.
Which is ridiculous for a 6th level character, frankly. But there you have it.