It does seem like he is struggling more to get the nose in on the 600LT, and hold it in mid corner and that is likely tyres which seem to make more noise, based on a quick watch. 600LT picks up speed more quickly and both hitting 300ish under the bridge on the main straight is incredible not least as he carries that speed under the bridge. Merc is more settled and think I said before it does flatter the driver on track but can see how that would make the 600Lt more of a challenge.
Interesting for me is how well the Merc held on to the GT3 race car (could not make out if it was another GTR or Z4) car for a bit, quicker a touch on the straight bits but gobbled up under braking and left, right, left type affairs as expected. Two great cars getting a similar result in different ways.