I have slotted/drilled rotors mainly for looks. My OEM rotors were warped. It seems like there is a lot more grip when braking from various speeds. Even after having them for about 4k miles. I have the Nakamodo brand.
Well if you're comparing to warped rotors, then anything will give better grip. Warped rotors are a nightmare, not just by themselves, but the vibrations cause excessive wear to control arms, bushings, chassis components, and so on. Warped rotors are really a pain in the arse and they're enough to drive people like me completely mad.
If you're referring to grip as in braking power, the slots would account for some of that. The cross-drilling isn't giving you any measureable improvement versus a solid rotor, but the slots will definitely help, especially once the brakes get hot. Initial bite from cold brakes is always pretty solid, but once they heat up and stay at an elevated temperature, they lose bite because of the gas layer forming on the surface. Slots provide an escape for the gas, so you get better pad/rotor contact any time you apply the brakes.
The problem with drilled rotors isn't the fact that they don't improve performance, and the initial performance may be just as good, but they heat up more quickly, so the need for slots is even greater. This is, of course, assuming we're comparing rotors that are identical in material, vane geometry, size, vehicle, etc. You definitely want to torque your wheel bolts properly and check them every month or two. I've done a lot of research on warped rotors and while unevenly torque wheel bolts are the main cause, they don't usually warp to where there is measureable runout. The warpage is in the material itself and how it changes when it heats up. Hot spots and cold spots so to speak, which changes how it engages the brake pads. This is usually caused by a combination of heating and cooling paired with uneven pressure from the wheel bolts, and since they heat up more quickly, they're more prone to warpage as well as stress cracks (which will cause changes in shape and dimensions). Just my two cents on how to maintain them as well as possible.