Congrats on the car! If you ordered the optional larger brakes, your M2C will require 19" wheels. If you opted for the standard brakes, read on, as you'll have many factors to consider!
Everyone has their own priority list, so these factors mean different things to different people. To break things down a bit:
Performance- Wheel & Tire Weight - Larger diameter wheels are inevitably heavier. Larger diameter tires with the same overall rolling diameter are often lighter. However, our rule of thumb is to run the smallest diameter possible that still clears the brakes. With the M2C, the larger brakes are going to require you to run 19" wheels, whereas the OG M2 can happily get away with 18s.
- Tire Availability - Not necessarily an issue for this chassis, but in some cases, it's harder to find wide (305+) low-profile 18" tires than it is to find 19" tires (like the Cup 2s you mentioned for example).
- Wheel Width & Offset - You of course want to make sure everything fits with as little modification (read: expense) as possible. Stock wheels are made to fit and work under all circumstances, however it is always possible to squeeze just a little more grip from your setup by adding wider wheels with different offisets, dialing in negative camber, rolling fenders to create space for wider tires, etc.
Saftey
For every legitimately engineered, tested, and properly rated wheel out there, there are 10 that aren't. It is important to make sure your track (and street) wheels carry a sufficient load rating to cope with the demands of track use such as sitckier-than-stock tires, increased power, additional downforce modifications, and more aggressive brake compounds. All these factors can 10x the amount of stress placed on a wheel, and you should make sure they are up to the task.
Aesthetics
Do you want the lightest possible setup? Or do you want to make sure you close that wheel gap just right? For some, track days and HPDEs are just something fun to do every now and then, while others are out there chasing tenths of a second.
Cost
Smaller wheels cost less. Smaller tires also cost less. Wheels & tires become consumables when used on track regularly. Get where we're going?
With all that said, the 763M wheels are a fantastic choice for track use, and are surprisingly light for OEM wheels. Our only hesitation would be the risk of damage, in which case they are quite costly to replace. Aside from that, they'll look phenomenal and should serve you well!
--Dylan