This is a known problem with many cars. In some cases you can find the correct ratio online and then apply it to the car using ballast and that will make this right. It isn't a case of reversal. For example, the RUF BTR is listed as 40/60, whereas that should be more like 38/62. The Alpine A110 on the other hand, another RR car is listed as 50/50, which is totally wrong and sure as heck explains why that car just doesn't handle like an RR car when in online mode (offline it's pretty good even with the incorrect weight balance). In order to make that car have the correct weight ratio (I was unable to find it online, but it is supposedly a bit more neutral than a 911, so I set mine to 40/60) you have to add a bunch of weight to the rear, but it drives much more appropriately (online) after that.
At any rate, I have edited a number of my cars which I discovered were incorrect. Even if those cars handled pretty nicely offline, I did find that the cars behaved more like they should in online mode once I set their weight balance to be correct. So IMHO the best bet is just to go through and fix it for those cars which matter to you.