A lightning claymore +5 does 257/257 = 514 damage and can be wielded for full damage at SL 7.
A standard claymore +15 does 322 physical damage at base required stats. In order to match lightning +5's damage you would need 77 combined STR/DEX (513 damage) which puts you at SL 55 if you ONLY put points into STR/DEX, leaving you with base VIT/END.
With the lightning claymore by SL 54 you could have 40 END and 30 VIT with 1 stat point to spare while still matching the standard's damage (not spending that last spare point and being a lower SL means you can invade the standard too).
All of this is after the 1.06 elemental weapon nerf btw, it's also worth pointing out the dual def check penalty of multi-element weapons is much less significant in PvP than NG+ PvE since character DEF levels off much earlier.
(assuming both chars are using Havel's ring):
The standard+15:
-513 physical damage
-can't deal damage thru a decent shield
-base VIT/HP
-only enough stamina for 3-4 swings max
-not enough equip weight for any decent amount of def/poise without losing fast-roll
The lightning+5:
-514 physical/lightning damage
-can deal damage thru any shield
-almost twice as much HP as the standard
-max stamina bar
-enough equip weight for much better def/poise while maintaining fast-roll
or
-highly superior def/poise with mid-roll
Now obviously no one in their right mind would create a build like the standard but the point is that the requirements to achieve similar damage levels with a standard weapon compared to an elemental weapon are massive.