Well, I'm of the firm believe that alloy diff cases are also a necessity on the savage. At least with the old savage style diff case/bulkhead design. Over time, the plastic fatigues and distorts which lets your ring/pinion mesh get out of sync and tears up your gears.
While inner cups being alloy helps with spider gear issues, they do very little for the ring/pinion mesh. However, the XL (possibly X as well) cups use little chromed hardened steel pieces for the cross pins to spin in, so they aren't digging into the sides of the alloy cups. A happy surprise for me when I just redid mine.
I just upgraded both of my diffs to hot racing diff cases, XL diff cups/internals and XL ring/pinion gears. My outdrive cups however must remain old savage as they are smaller. The larger ones on the savage XL don't fit through the hot racing cases. Kind of irritating... but, hopefully I won't be messing with my diffs for a very long time.
For the rest of the rig, the chassis braces that the lower skid plates bolt to should be replaced with alloy as well.
Honestly though, the diffs have always been my only real sore spot with the savage line. But, with the upgrades I've done, I'm hoping I will have as tough as a machine as my aftershock has been as far as the drive train is concerned.
As far as roll cages are concerned, protecting the engine is your only real issue. The RCS cage will protect it, but for the money, I'd buy 2 or 3 welded round bar cages. The RCS cage bends and fatigues, then breaks after a few bad hits. They do their job, but you can't replace individual pieces when they break. So, spending your money on a RC raven cage or something similar will be money spent just as well.
Personally, I make my own roll bars for all my rigs if I can. They hold up better than any cage/roll bar I've ever bought. They just aren't as pretty!

Take a look and form your own opinion though:
https://www.rcnitrotalk.com/forum/showthread.php?t=63999
This is my RCS cage on my savage from 3 years ago:
http://home.comcast.net/~98gmarquee/pics/misc/2005-0917-BentRCSCage.JPG
Like I said, it did it's job, but when I tried to straighten it back out, it snapped in a couple places.