Ok so I just got my first 90, I've had 2 81s and a 82 b4. This is an entirely different beast.
Love the car, thing is smoking fast. got about 5oook on a fresh rebuild. Bone stock. Had it into the dealer they recognized it immediately as they have serviced it all its life, YOHOO!
Its in cherry condition, bone stock, but am wondering about normal engine temperature. The dash guage reads at the half way mark till I go up a long hill or race it for even only a few blocks in the city, then it goes up tiny bit more.
I've read about stock cooling is best, what about a performance rad?
I've added watter wetter and dont notice a difference.
Should I add a aftermarket guage?

 

The S5 gauge normally reads at the halfway mark.
Whether you want/need an aftermarket gauge depends on your level of paranoia.

 

This should give you an idea of operating temperatures once you install that aftermarket gauge. https://www.rx7club.com/2nd-generation-specific-1986-1992-17/what-too-high-water-temp-663516/

I never saw the gauge in my S5 surpass the halfway point but I did install an aftermarket gauge just to get an idea of what my temp reading was.

 

My S5 Turbo runs at 92 from what my Microtech dash unit says my gauge never passes the half, stays at around 1/4 or slightly above. (I need a new rad though cuz mine is for the original engine, s4 86 NA and cant handle the it, doesn't help I don't have a fill cap either). As far as I know the stock gauge works just fine. On my old engine it showed me a HUGE difference from just changing the thermo, went from just a bit over the 1/4 to just bellow it like 1/8ish. So yeah no point in going aftermarket if yours aint broke, unless your going for a digital one that says the actual number instead of the pointer gauges, mine came with my aftermarket ECU.