I've always like Metzeler tires and don't have any issues with the OEM tires. Honestly, there aren't many choices in the size range for our scooters. Fortunately, they are the same size tires used by the Yamaha TMAX but I'm not aware of any other machine that uses those sizes. And our scooters are considerably heavier than the TMAX so what works well for that scooter may NOT work as well as on our machines. You'll see plenty of differing opinions here and I'm sure others will chime in shortly.
By the way, I have a 2013 model. If you're looking at the 2016 and newer models, you may not even NEED the Dr. Pulley Sliders and HIT Clutch as BMW apparently addressed the issues that made owners of the earlier models look at those items. I haven't been able to find a confirmed answer as to whether or not those new parts are retro-fittable to the older models so aftermarket may be my only answer. To be perfectly honest, though... I don't have any problem with the performance of my scooter off the line. I've always been easily able to safely pass slower traffic without any issues.