Mana is definately a thing to be considered... I think you might be better off spending a second spoil on one in 10 mobs than using 5 more mana on each.
On the other hand I never deliberately deleveled any of my chars. So what do I know... good luck with your business tho. :) Hope you make lots of money.
Cal
Cal has it right.
If you level to 28, do not take your new spoil skill before you delevel.
For starters, it doesn't actually increase your chance of spoil landing or the mobs turning blue. What it does do though is increase your MP consumption by alot, and since you will have MP max and regen of a lvl 20 toon, this will be bad for you.
It is good to take all of your other lvl 28 skills because the extra masteries will help you kill faster and longer.
Typically I find that if the mobs you are spoiling are white to you, spoil lands almost 100% of the time. Whether or not the mob turns blue is a factor of the spoil rate of the items that mob spoils. There is nothing you can really do to increase the rate of the mobs turning blue. You can decrease it though.
If you spoil dark blue mobs, spoil will land about 100% of the time, but you will find the mobs pretty much never turn blue.
Typically the higher the mob is above your level, the lower the chance of spoil actually landing. (you know the message that the mob has been spoiled.) I find that white or yellow mobs spoil close to 100%, pink is prolly 80%ish, after that, the percent of spoil drops way off.
In a nutshell, if you have a target mob that you want to spoil, you will get the best results if you are +/- 6 levels to that mob. ie.. it is between green and yellow to you.
Also worth noting, there are all kinds of rumors about buffs that make your spoil land better. They are all BS. There is one buff that affects spoil in any way, and that is haste. If you have haste, in any of it's forms including potions, it will make your spoil cast faster, and the reuse timer will be shortened. I believe atk speed is related to how fast you cast spoil and how long the reuse is on it.
Happy Spoiling!!