It's intended to be used as an opener or a way to knock a guard out while sneaking around. But yea, if you wish to ignore the intent, that looks about right.
Keep in mind, he can't do it with any Sneak Attack. Sap master requires the target to be Flat Footed, which is a very specific condition that is difficult to obtain beyond the opening shot.
If you don't enforce this condition those feats are way over the top.
Due to some of the authors using the term "flat-footed" when they meant "denied their dexterity", it's not so hard.
You should never power attack (PA) with sap master, and probably not with normal sneak attack (SA), either. Once your other damage sources (besides power attack) reach a sizeable amount, the decreased chance of hitting actually will lower your damage output more than the PA damage boost helps it. Even just a -2 penalty can be devastating. TWF and Rapid Shot are a different story, since for their -2's, you're getting an entire new attack. PA is not like that, it's just a straight up accuracy for damage swap.
Say with Sap Adept/Master, I can deal 6d6+12 (ie, level 5+ rogue). I'm not even going to factor inbase weapon, enhancement bonus, or any other damage sources, just the raw SA damage. I'm going to use a level 6 rogue, so he has BAB +4 for a -2 PA penalty. Say he would hit the foe with his first attack 75% of the time w/o PA. Heck, any % below 90% or above 10% it would be the same case. His % of hitting goes down by 10% with PA.
6d6+12 averages 33 damage.
PA would be adding 6 to that for a 10% lower hit rate. (1.0-0.1)*(33+6) = (.9)*39 = 29.7
Look what happened there! Your damage output actually went down by a little over 3 points! As the PA attack penalty grows and the SA continues to boom at +2d6+2 every other rogue level (along w/ other damage bonuses like enhancement climbing), this trend will only get worse. Let's take a snap shot of Rogue level 12. He has SA +6d6, so w/ Sap Adept/Master, it's 12d6+24, average damage of 66.
His BAB is +9, so PA is -3 to hit for +9. That's 15% hit rate decrease.
(1-0.15)*(66+9) = (.85)*75 = 63.75
Yes, when your have almost no chance of hitting or missing away, the PA is better. But those are either fights you have no hope of winning or fights you could breeze your way through. Why are you going to waste a feat on such fights? I'd be more worried about the ones that are actually challenging, not over- or under-challenging.
If you don't enforce this condition those feats are way over the top.
I disagree. Rogue is very weak, letting them use this on "lost dex" instead of just "flatfooted" would still not make them anywhere near as powerful as a spellcaster, and as long as your game allows wizards and clerics and such, then that is the "it's broken" threshold a class must surpass to become problematic.
You have some very strange and dangerous misconceptions about this game.
Sure, Rogue is somewhat weak.
But if it worked on just "Lost Dex", that ninja at level 10 with improved vanish is going to TWF full attack for XXX + 10d6+10, 4 or 5 times - damn near every round.
50d6+50 from raw sneak attack at level 10 on a full attack (plus whatever else from weapons, strength, etc..) is a bit silly for any class.
The feat chain is designed to mimic someone sneaking up and getting one or two really solid hits to knock them out. The intent was not to steamroll someone uncontious mid-fight.
You have some very strange and dangerous misconceptions about this game.
Like what? Do casters not have save or dies to throw around, battlefield control to stop monsters in their tracks, powerful buffs like haste, debuffs like color spray to take a foe out of the fight for some time, and more out of combat utility spells than a calendar has days of the year? If so, that's news to me.
What if you took the scout archetype?
Link
Scout should probably work with sap adept/master RAW. Maybe not RAI.