Whats your definition of "much larger"? The pvp set bonus does 3-4% more dmg.
0.15(increased crit chance) * 76 (surge) = 11.4%.
Whereas the pve set bonus gives you a flat 8% increase.
That math is incorrect. You are forgetting that the critical bonus damage is increased by an extra 30% with firebug skill. So it would do .15(106) - 8 = 7.9% Taking into account rail shot puts up about 25-30% of your overall damage, it is about a 2.3% increase in overall damage. I would say that is significant.