OOP vs Inline with Arduino
The reason you can't find the answer is because the answer is both Yes and No.

For basic class stuff - defining your class with methods etc and instantiating objects from it - there's little difference in the end result compared with "vanilla" C.  The compiler's optimizations are so good now that performance is just the same. Yes, there may be slight increases in memory use since you're passing an extra pointer with each method call (instead of foo(int x) you have foo(MyClass *this, int x)) but that's so small as to not be noticeable.

The big differences come when you start playing with polymorphism and other advanced topics.  When start doing these complex programs the compiler is not always unable to work out which functions are required and which aren't, and it's not able to cut out the unused functions ("garbage collection"). So you may well end up with bigger code.

That doesn't mean slower code, just chunks of code that are hanging around that never do anything.

Of bigger importance is managing your dynamic memory better than you are used to.  Since there is such a small amount of memory the heap is very small, and as a result it gets fragmented very easily.  Dynamic creation and destruction of objects (new myClass, delete myClassObject, etc) is very bad.  Class objects really need to either be statically defined (in global scope is the most common) or temporarily allocated on the stack (local instances).  Otherwise you're asking for trouble - and the first you will know of it is strange things happening (no error reports or exceptions, you see...).