I don't know if I really need to say this but when I ask something like what is better about some aftermarket part than stock I'm just trying to understand what parts might actually make my D413 better.
I'm not big on bling and it seems like that's what many of the parts available for this buggy are right now. Below is what I've seen that would seem to make a functional improvement. I'd love to hear other opinions though if I'm not seeing something.
Ball bearing ackerman bar from RCShox or Exotek.
Exotek machined spur and mounting plate.
Machined shock pistions.
Maybe the rear clamping hexes from Exotek. I used a bit of grease on my pins and haven't had one fall out a single time but I can see why someone would pick these up.
Titanium turn buckles. I have these even though I haven't bent a stock one.
The longer Shelle chassis for certain track conditions.
The Avid chassis if you are already adding weight to your stock chassis and want to keep that weight as low as possible.