Here's how it really works. I've done extensive testing (I'll include my notes at the end of this), and it's fairly obvious, once you get past a few hurdles.
If this has already been posted before, I'm sorry, I didn't see it. At any rate, an additional post on the subject couldn't hurt, considering how many are misinformed about the topic.
Every time you receive a new Store TP job trait, your skill is set to a certain level. These are the levels:
Level 10 Samurai: Store TP = 10
Level 30 Samurai: Store TP = 15
Level 50 Samurai: Store TP = 20
Level 70 Samurai: Store TP = 25 [I haven't been able to confirm this, personally, but I've heard convincing reports toward it - feel free to experiment]
None of these are nerfed in any way by subbing SAM. It's exactly the same (proportionate to your SAM level, of course).
Whenever you equip something with a Store TP+ value on it, that value goes toward your skill (obvious enough).
So, if you're a level 50 Samurai and you equip the Shinimusha Haidata, you have Store TP at 20+6=26. If you equip both the Haidate and Hara-ate, you have Store TP at 20+9+6=35. If you're a WAR/SAM at level 30 equipping the Katana Obi, you get 10+1=11. Simple enough.
Whenever you hit something, or you get hit (yes, Store TP does affect TP gained when hit, too, though it's obviously much less noticable), you get a base TP gain depending on your weapon's delay (or a flat 2.1% if getting hit - this is unaffected by your weapon). Here's a few base values:
Getting Hit: 2.1%
Delay 195: 5.3%
Delay 236: 6.3%
Delay 450: 11.3%
Delay 504: 12.3%
Assuming you have Store TP (ie. you're a level 10 Samurai, or a 20+ job subbing Samurai), you'll get an additional percentage of the base TP. This percentage is, surprise, your Store TP value. If you have Store TP at 20, you get an additional 20%. If you have Store TP at 35, you get an additional 35%.
A few more examples, in the case of a level 50 Samurai wearing the Shinimusha Haidate (Store TP = 26):
Getting Hit: 2.6% (base 2.1%)
Delay 195: 6.6% (base 5.3%)
Delay 236: 3.9% (base 6.3%)
Delay 450: 14.2% (base 11.3%)
Obviously, the higher the delay of your weapon, the greater the effect of Store TP.
What this means is that with a full Shinimusha set and a 450 Delay weapon, at level 50-69, you'll be gaining 15.2% TP per hit, as opposed to 13.5% without the set. You'll also gain 2.8% every time you get hit, as opposed to 2.5%.
That may not seem like a lot at first, but add it up through a few rounds, and it's a very noticable difference. Not Godly in any way, no, but certainly better than what most people describe the Shinimusha set as: Useless.
Below are my test notes. They're not very pretty (even less so here than in my text file), as they were meant only for myself, but at least they should indicate that I'm not pulling these numbers out of my ass. Peruse at will.
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29THF,D195: 5 10 15 21 26 31 (33) 39 44 49 55 60 (62) 67 73 78 83 89 94 99 104 = 5.3%
30WAR,D195: 5 10 15 21 26 31 - 42 47 53 (55) - 65 71 76 81 86 92 97 102 = 5.3%
42SAM,D195: 6 12 18 24 30 36 42 48 54 60 66 72 78 84 90 96 102 = 6.0%
52SAM,D195: 6 12 18 25 31 37 = 6.3%
52SAM,D195: 6 13 19 26 33 39 46 52 59 66 72 79 85 92 99 105 = 6.6% (Store TP+6)
29THF,D236: 6 12 (14) 21 27 33 39 46 52 58 65 71 77 84 90 96 102 = 6.3%
14SAM,D236: 6 13 20 27 34 (36) 43 50 57 (59) 66 73 = 6.9%
15SAM,D236: 6 13 20 27 - 41 = 6.9%
15SAM,D236: 6 13 20 27 34 41 48 = 6.9% (Store TP+1)
42SAM,D236: 7 14 21 28 36 43 50 57 64 72 79 86 93 100 108 115 122 = 7.2%
52SAM,D236: 7 15 22 30 37 45 = 7.5%
52SAM,D236: 7 15 23 31 39 47 55 63 71 = 7.9% (Store TP+6)
No Store TP (theoretical), D450: = 11.3%
42SAM,D450: 12 25 38 51 64 77 90 103 116 129 141 = 12.9%
52SAM,D450: 13 27 40 54 67 81 = 13.5%
52SAM,D450: 14 28 42 56 71 = 14.2% (Store TP+6)
30WAR,D504: 12 24 36 49 61 73 86 98 110 = 12.3%
15SAM,D504: 13 27 40 54 67 81 94 108 = 13.5%
15SAM,D504: 13 27 40 54 68 81 95 108 = 13.6% (Store TP+1)
42SAM,Def D195: 2 4 7 9 12 14 16 19 21 24 26 = 2.4%
29THF,Def D236: 2 4 6 8 10 12 14 16 18 21 23 25 27 29 31 33 35 37 39 42 44 46 = 2.1%
14SAM,Def D236: 2 4 6 9 11 = 2.3%
15SAM,Def D504: 2 4 6 9 11 = 2.3%
42SAM,Def D236: 2 4 7 9 12 14 16 19 21 24 26 28 31 33 36 = 2.4%
42SAM,Def D450: 2 4 7 9 12 14 16 19 21 24 26 = 2.4%
52SAM,Def D450: 2 5 7 10 12 = 2.5%
52SAM,Def D450: 2 5 7 10 13 15 18 20 = 2.6% (Store TP+6)