I don't know where you read/heard about Darth Vader's suit being an *upgrade* to General Grievous.

In [Wookieepedia][1], the section for [Darth Vader's armor: Discomforts and limitations][2]:

> Having had so much experience with mechanics, Vader was dismayed by the incompetence of the medical droids responsible for his resurrection in Sidious' laboratory on Coruscant. **The technology in the suit was already obsolete, having been used to rebuild and create General Grievous decades earlier.** While he retained his knowledge of the Jedi arts, he began to feel uncertain about his place in the Force, and while he had taken his first steps toward awakening the power of the dark side, he felt uncertain about his ability to sustain that power. He knew that he was hampered by his injuries, and knew he never could achieve his full potential because of it. **At times, Vader wondered if Darth Sidious had purposefully designed the armor to restrict him.**

It doesn't seem to be considered an upgrade at all.

  [1]: http://starwars.wikia.com/wiki/Main_Page
  [2]: http://starwars.wikia.com/wiki/Darth_Vader%27s_armor