Notice that it says that the bonus damage from stealth is "per skill point" not based on effective skill.  That may help make it clearer for you. (epeli corrects this, below)
edit: oops, sorry, first part of your question, too.  326% is the correct number for your bonus damage.  The "bonus damage" is what's on top of 225% from your stealth.
You can probably test this out on the target dummies in SGS lvl2 and see the numbers for yourself. Save, fire off a dozen rounds or so, and you should see you're doing (base damage * skill damage modifier * 3.26) - give or take a little for additional modifiers.
I have tested this in the SGS target range now, for science!
Sniped the target 30 times, recorded the result.
Edit: Summary/conclusion:Snipe does not work like a critical hit. The total Snipe % as shown in the tooltip is a direct adjustment of the regular damage, not extra damage on top like a crit. For example: 300% snipe damage means triple damage, plain and simple.
(This paragraph in the wiki: "
0.2.0.1 - fixed the bug that caused the attack not to do those base 100% of damage in addition to bonus damage" does not make any sense. There is no bonus damage.)
Info:Guns skill: 50 (75)
Stealth skill: 50 (110)
12.7mm Harbinger Bipod, damage 63-95, skill adjusted damage: 
96-143Snipe damage as given by in-game tooltip = 
335%(So feat description is correct, 225% + 110% (effective stealth skill) = 335%)
Expected snipe damage if formula is normal damage * 335% =
322 - 481 (average 402)Expected snipe damage if formula is normal damage + 335% extra damage (like a critical hit) = 
419 - 625 (average 522)Result:452   395   465
455   398   405
425   361   388
351   469   479
462   358   368
469   375   398
378   345   358
455   368   345
381   335   341
331   455   361      
low   331   
high   479   
avg   398   
Normal damage * 335% = 322 - 481 (average 402) seems correct.