I really can't comment too much on it. I haven't done much overall in the GT3 class personally, and almost nothign with that car in particular. I am running GTE in our league. We are running and IMSA style series with LMP2, GTE and GT3.
In both GTE and GT3, the 488 WAS by far the clear runaway car. Unfortunately, no one is driving the 650 in the series, so we don't have a lot of hard data on it to compare to, other than the data we gathered for the HP loss at altitude testing. From conversations with other drivers though, the 650 just doesn't handle as well as the 488. In both classes (GTE/GT3) the 488 not only seemed to have a power advantage, but it also had a near perfect tune on it right out of the box. It was just a straight up perfect car. Anyone with even moderate driving skill could jump into it, never touch the default setup, and run at the front.
I do know that the turbo cars have a definite power advantage right now. To the point where there is no reason to even consider a non-turbo car in either class.
Since patch 3 came out, the 488 has been toned down a little bit, but it's still a clear front runner. The changes reduced the power, and impacted it's high speed cornering, but then they also improved it's low speed cornering as part of it, so overall, while it did drop in pace, it didn't drop a whole lot. We are currently prepping for our race on Indy Road Course tomorrow night, and from what I have seen, the car is about .3 - .5 seconds per lap slower there than pre-patch.
I think the update for the non-turbo cars will close the gaps a lot more. As you can see in that thread I linked, the non-turbo cars were getting hit pretty hard by the altitude change (double to triple what they should be). The Corvette C7.R and AMR Vantage GTE are currently both losing ~40HP for a less than 1000ft change in altitude. Based on our tests, at Nurburgring (highest elevation track in the game), it's about an 80HP loss, while the turbos lose almost nothing. That's probably the main reason why most of the cars fall so far behind the 488, although it doesn't explain the rest of the turbo cars in GT3 also falling behind (Bentley, Cadillac, GTR, NSX, etc.).
I'm confident that with the changes in Patch 3, and the eventual patch 4 update, that they will get the fields balanced much better.
I'm just really happy to know that they are listening, investigating and addressing the concerns.