Summoner Damage Calculator

[Dwill]

So using Spur is like re-summoning it with 20 sec of 40% dmg buff.

That's half-true (unless I'm not understanding correctly). After the 20 sec, the pet keeps the snapshots from Spur, it won't revert back to what you first summoned it with.

Since Spur takes into account Physical and Magical attack power, it needs to snapshot what these two values are in case they changed over the encounter. An easy way to see it is, is like Madoka suggested, just summon Titan using a full Vitality set, switch back to your actual DPS gear then use Spur. You'll see his max HP take a huge drop. It works the same way with Int, Accuracy, P&M attack power, etc.

Edit: If you need to test in a combat situation, it's quite easy to test as well. Go to a dummy, get naked then summon any Egi, put back your DPS gear then send the Egi on the dummy via Obey. Note the numbers after the Egi whacks at it a few times. Then use Spur and see those numbers rise up drastically.