Feign Death, L16 - 1.5 sec cast, 60 mana, 15 second recast
Comatose, L52 - 1.5 sec cast, 80 mana, 8 second recast
Death Peace, L60 - 1.0 sec cast, 120 mana, 4 second recast
As you can see, the trend is an increasing mana cost, shorter cast time, and most importantly, the recast time cuts in half each time.
When you get comatose, you can have both it and the original feign death in your spell lineup if you chose, as the recast are all on different timers. When you get to death peace, the recast is short enough that the separate timer isn't really a factor.
At L66, you can get the AA version of Death Peace that pretty much takes the recast out of the picture. You can also use it during your spell gem refresh, which means you can pretty much drop agro at will, allowing you to go max dps with very little danger.
I should warn you however, all levels of feign death still occasionally give you the "fallen down" message, so you'll still have to pay attention. The later levels are much more surviable due to the shorter recast, however.
Hope this helped.