It all depends on what you want and how you shift and launch.
Steel shift forks were a must for me after breaking so many of the stock aluminum ones. But on the other hand many others have done all kinds of hard shifting on the stock ones without ever breaking them.
See spy's post for a good sum up of the GVR4 1st.
The double synchro second is a good upgrade, but I'm not sure whether its worth the money or not. I had it installed in my old fwd and did notice a small difference in the ease of throwing her into 2nd at high RPM. However, I don't have it installed in my awd and have yet to have any problems shifting it into 2nd even at 8500 RPM's, but then again previous broken dsm trannies have taught me to be a much gentler shifter than I used to be. YMMV.
I have yet to hear of anyone around here breaking a stock shift rail, so I figure that one can go under the 'if it ain't broke don't fix it' category.
Finally, if you truly do have a 4-spider diff, and if it truly is broke, I doubt you broke it due to power output. Darryl has run numerous 10 and 9 second runs on his speed designs 4-spider with no problems whatsoever. Maybe yours was machined improperly and therefore prolly not a speed designs diff. Or maybe its not broke. Or maybe its not a 4-spider diff.