I can't add more weight to both cars. I ran it at 512hp and 1345KG according to the spec. However, the #91 churns out 607pp instead of 605pp like the #51.
After a few laps on stock condition (no suspension, LSD setup) the #91 is more or less a second faster. The handling is pretty much the same although the #91 feels more nimbler, however the straight line speed is the big difference. The #91 is very fast on the straight, even considerably faster than the R8.
Both are very good managing its tires, although in my driving, the front wears first than the rear. But the tire saving is similar to the mighty GT-R. Yes, its that good people.
All in all, the car is easy to drive and most importantly the sound is very sexy.