Alex, the car is completely stock excepted the springs. So yes, the XB8R has the softer, longer suspension arms.

I would NOT say that the XB8R is "by far less durable", it's just not as fancy. My friend's car has survived more than 7 liters (~2 gallons) of abuse on the track and hasn't shown a weakness yet.
The engine mounts and shock towers do their job just as well as the expensively-machined XB8 parts. Yes, some might even say the XB8R is better and this is likely to be true for some tracks. If not, why would the Xray upgrade kit for the European Championship include the softer arms and composite braces?
Time will tell if the composite suspension blocks are tough enough.