Tires are maintenance. Choosing a different set when replacing a worn set is upkeep. Running a spare set for racing is a mod.
However... Wheels would be the mod I would choose, specifically forged wheels. Better style, lighter rotational and unsprung weight.
Single mod, but improves power by requiring less energy to move a lighter rotational mass, suspension similarly with less unsprung weight that allows springs and dampers to work better, and usually the most significant single thing you can do to upgrade a car's looks.
With a Z35, I think the natural choice would be the Volk TE037.
Rays has history with the Z34 and other Nissans, and shares a Japanese automotive aesthetic.
The TE037 is a modernized update of a classic design, the TE37, like the Z is also.
Usually odd-spokes look better, but 6 spokes is symmetrical, and coincides with a tradition of 6 cylinders.
It is a concave deep wheel, which emphasizes RWD offsets, and possibly staggered widths.
with spokes right to the edge, which looks better and more modern on modern cars.
It is not directional, which means that it doesn't need mirrored versions, or the opposite sides of the car don't match.
It is forged and light weight, for the reasons above
It is strong for street use, not an ultra-light delicate race wheel that would not stand up to bad pavement.
It comes in hyper silver/smoke, bronze, or black, which suits most colors and tastes.