Don't worry on using wheel spacers. Honda engineers are fully aware of the abuse Type R see and are designed accordingly.
Why I state this? The DE5 HRC has +45 Wheels that equal to a 15mm spacer...
Will the dynamics of the car change? Yes. Can you feel them? Perhaps, I don't know your driving...
Honda won't spend ingineering time and cover + bolts for nothing.
Looks to me the cavity is for coolant fluid... to keep it temp control since is over the exhaust manifold.
I understand Honda not to include a loud exhaust. Most of us will change the exhaust system anyways...as corporation, might as well meet regulations and let the aftermarket deal with loud mufflers.