I am posting this finding on KI, BG and Alla, so if you have read this already, skip.
I have recently tried to create an excel damage calculator for gear comparison purpose that account for all phases of the game, upon doing so, and knowing close to all game mechanic, I couldn't understand why Spinning slash, ground strike, gekko, steel cyclone, howling fist and some of the other one hit WSs are doing more than they should. If you use the KI damage calculator for those ws, or simply use the known ws damage formula and calculate it yourself, they will always suggest a lower damage than you see in game, and I believe that most ppl speculate that there is either a hidden attack bonus or a straight up pdif % bonus. In order to make my damage calculator extremely accurate in comparing what are the better overall weapons for each jobs, I figure that I have to try to narrow the hidden trait for those ws in order for the results to be accurate.
The two possibilities in here are that those WSs either give a straight attack bonus (100, 200, or 300 etc), or give a % of user's pdif (your attack/mob's def) before adjusting for lvl correlation. Therefore, in order to test this, it is best to find mobs that are hard enough so you don't cap your cratio, and one that you can have a large sample, and one that only have minimal lvl difference between the mobs, and most important is that we need to know the stats of the mob. To fit all of the above, meriting in bird camp seems like the best way to test this. Note that from some of the testing that I took from Alla.com SAM forum, they seem to suggest that it is either a flat 200 attack bonus, or a 40% pdif jump.
So during the weekend, I merited in the setup of drk/sam, war/nin, war/nin, mnk/nin, brd, rdm at lower bird camp for about 230 mobs. Songs were march and min (I believe the min gave me around 59 attack instead of the capped 66), 50% of the time I would have hasso on, no SE, no dia (I told the rdm not to), no food, and with the following WS setup, hume, 5/5 str, 6 hit build: http://ffxi.allakhazam.com/profile.xml?182779
Under the condition, my calculator suggested a 830 spinning slash average assuming a straight 200 attack bonus, accounting DA (not account for missed DA), exceeding tp over 100% which will result in slightly stronger WSs (the calculator suggested an average of 109% tp when I fire a WS, this is assuming 5% human error on not wsing when having 100%, and obviously meditate will screw with your tp, which make it impossible to always at 100% tp when using ws). I am a pretty aggressive player so I would say that I probably WS at a even lower average than 109% tp, and it also suggested a 900ish spinning slash average assuming a 40% pdif bonus. So the difference is pretty big under this condition so the result should point toward one or another.
After about 185 ish ws, my parser showed a 820ish spinning slash average, which included 1 that hit for only 305 that missed the first hit but connected the extra hit, which slightly brought down the average since my calculator doesn't account for that, and I also had less DA than 7%, so I would say that the sample might be on the lower end in term of average damage.
So from the look of the parse result, which is a bit on the lower end than my past parse results (probably due to having dia and the fact that I use SE with SS sometimes when I merit in normal condition), it suggest that it should be a 200 attack bonus instead of a % pdif increase.
I will post the pic of my calculator and parse result if you would like; I am at work now so it will have to wait til I get home. But despite how small the data is, IMO it is pretty clear that it is at least not a % increase. And combining with some of the data from SAM forum, 200 seems to be correct.
This explains why Spinning slash, gekko, and ground strike are all so great against HNM, the most obvious is that despite a higher ftp, spiral hell doesn't perform as well as ground strike when both are at 300%. And this also shows why they don't perform nearly as well as multi hit WSs when mobs are lower defense, which decrease the value of the 200 attack bonus.
Just to illustrate it a bit more. This finding is important in selecting gears for WS as it shows that attack is less important. Ppl all know to do this but there is never a good explaination (spinning slash is only 30% str and 30% int, so why we stack on str instead of both str and attack). And for it not being a % increase in pdif means that by putting 10 attack, it won't become 14 attack, so it further decrease the value of attack in such WSs. So yea, I hope this finding helps your gear selection.
Also note that this doesn't mean that all attack bonus are going to be the same, but it is assume to all be 200 unless you can test some of the other ws like howling fist, steel cyclone etc. At this point, it seems like Spinning slash and gekko are both 200 attack.