I have been in talks with a roll cage manufacturer but even the cheapest welded in version was silly expensive. If I am just to do track days I may look at making my own. I used to make them for my oval hotrods or worst case buy a second hand SD one and modify it. When I ran SEGTO hotrods the standard supplied cages were not enough for our regulations as we have to add bars etc. All I need to do is borrow an hydraulic pipe bender from a mate who still races. Not a five minute job as the bends need to be perfect in every plane, a bit of design work required. My only gripe with current cages is the door bars, I much prefer the curved ones that enter the door skin just kissing the inner skin as this gives more protection from intrusion, today's are angled as the skins and cards are generally still intact depending on regs.
As for other tweaks, I have a list but it costs alot of money so biding my time for items to come up and do it on the cheap! I'm currently hunting down some 4 pot calipers to beef the brakes up for track use. Suspension mods and a diff are on the list
But money is a luxury at the moment so as always I am having to be patient.