capture rates and damage are a bell curve I think.
like 100 damage/100 defense you deal 100% damage. 90 damage/100 defense you deal 80% damage, 50 damage/100 you deal 5%.
so you have to be in the level range to deal any damage to anything. its not so much +1% capture rate. its more about being within that 10% power diff range, so you have your 15% capture rate. when your 20% out of range, that 15% becomes 7%. 30% out its 1%, 40% out 0%.
I testd my Vanwyrm versus the helzypher.
Helzypher seems about 10-20% faster via stopwatch, but a solid 50% more stamina.
edit:derp. realized my helzypher had nimble. 10% move speed perk. so, speed deference is in question.