There already is a recent defiler AA discussion, just have a look and you'll get some nice points there.
I don't really see the point of locking at 50, progression or not progression, unless you do it for AA reasons.
You will be missing so many of your abilities, it's not even funny. You won't have spell crit gear, you won't have enough aa to unlock SF abilities, or even your stances.
I do agree we are slow to lvl regardless of spec and killing takes a while, but it's not really that bad, since you get a lot of xp anyway. Any class can be soloed to 90 these days, us included.You just have to live with it, accept the fact we're not really killing machines and just keep going.
Regarding dogdog, look at the descriptions in your STR tree. It's a chance to proc so it is not reliable. While there are items you can get to improve that ( funny this is discussed as well ), you wont be getting it at 50.
Get one of those AA mirrors when you can and use it for a solo / raid setup. Once you hit 90 and get some nicer gear from instances / raids, your spell crit will go up a lot.
If you need power you need to address that asap, get Cannibalise urgently , this is what keeps us going all the time , when many others are running oop.
Btw, powerleveling a defiler really is a bad idea, you need a lot of AA to improve your capabilities. I only have 178 AA atm and there loads of useful abilities I don't have. The stances are very useful , I use the healing one for groups / raids and the other for solo mode.