(no lucky powder can be used)
For every +, the maximum HP and MP increase by 1%.
From +3 to +5, the devil skill will change and give 25% damage increase as well as 15% speed increase. Also, mob defense ignore stat with 1% probability is added as a green (yes, green and not blue like on other equipment). The skill icon will change a little, but the character's look when using stays the same. You get an additional buff skill right to the devil skill which gives 300 parry ratio increase.
From +6 on, the damage increase is 30%, the speed increase is 20%. The mob defense ignore probability will raise to 2%. The parry ratio buff doesn't change, but the devil skill icon changes again and the character will look like this when using it:
More screenshots from cSRO site:
+5 devil spirit skill
+6 devil spirit with green mob defense ignore
Parry ratio increase buff
Source: http://silkroadsecrets.blogspot.com/
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....we are in Legend 5, i'll say JM is a bit late on this, when this comes out i'll spent at least 1000 silk in trying to get Devil Spirit S, then upgrade to Maximum/depending on how hard it is...so im guessing Spirit S +6 = 26% HP ( but looking at A grade SS it says 25% max upgrade so this means S will be 30% hp increase 
)increase+ heuksal passive. will probably give around 70K HP at lvl 110 
