I don't know if upgrade is necessary for E4JS. I have both E4RS and E4JS, I built them at the same time and always run both at the track. They have been on the track for more than 10 times and hit the wall pretty bad few times, but I didn't break anything. So it looks to me JS is as strong as RS. I would say the only major difference is one is plastic tub chassis another is carbon fiber, so JS will twist a little more than RS in corners. One thing I will try to improve is the sloppy feeling on both front and rear suspension linkage as well as steering linkage. I also noticed that JS has a little longer wheel base than RS by about 1.5 mm.