This isn't just a case of not knowing how to handle an old 911.
OVERVIEW OF GENERAL HANDLING ISSUES WITH CERTAIN CARS
There are borked cars in this game. They handle as you'd expect for the type of car / age / drivetrain initially, but when they start to corner above ~100 mph, all grip in the rear vanishes suddenly in a way that doesn't make sense for that car. It's as if a switch were flipped.
The old Alpine A110 does this. The Ferrari Dino does this. The Wicked Fabrication GT 51 does this. The BMW 3.0 CSL... The Superbird... Despite the era, suspension and drivetrain differences, they all lose grip the same way through high speed corners. The most straightforward fix is having as much downforce as possible at the rear and little or no downforce at the front.
You can experience this rear grip loss behavior at the opening corners of Trial Mountain, the first and last corners of High Speed Ring, and most of the Tokyo 600pp grind race when the track starts to dry out.
Other than downforce, you need to run softer tires on the rear, or mix compounds (ex: Sports Tires on the front with Racing Tires on the rear). If you don't, the only safe way to navigate high speed corners is with partial throttle, despite having mega grippy tires with a mega grippy tune.
Comfort tires and low speeds mask this problem.
INITIAL TEST OF THE PORSCHE CARRERA RS 2.7
STOCK
My initial test of the Porsche Carrera RS 2.7 was done all at High Speed Ring. Note that I run Dual Shock 4 with analog steering, braking and throttle. I am no E-sports level driver, but I do drive and tune A LOT. I've also golded all missions, licenses and tarmac circuit experiences; many by a large margin as well.
The first set of tests was through a range of tires, Comfort Softs (comes fitted to the car stock), Sports Hard, Sports Soft, Racing Hard and Racing Soft.
The Carrera RS 2.7 is manageable and lively on stock power across the tire ranges, but there's a disturbing hint of high speed looseness. Note that I never got above 160 mph. Most here would probably enjoy the Porsche this way, even find it exciting and engaging. But this early behavior tells me that the Porsche will suck for serious track work.
TUNED
I then did a near full tune - widebody, wide wheels, max weight reduction, engine, suspension, power, body rigidity, LSD, full customizable transmission, etc. I settled on a 600pp tune with Sports Hards and the low rpm turbo to see a worst case scenario.
For most of the track, the car was fine in that old 911 way - brake in straight line, balance the car with the throttle through medium speed turns... BUT when I tried to take the banked first corner flat at 170mph, the rear just broke away. I was able to manage it with partial throttle and steering corrections, but only just. Not fun. Not engaging. Just random, annoying and exhausting. Mind you, I love driving historic racecars in simulators! I even enjoy license S-10 in this game for heaven's sake!
The next 20+ laps I did everything I could think of aside from mixing compounds or having a high front / low rear ride height. I even put 200 kg of ballast in the nose of the car, changing the weight balance to 51:49 front/rear!!
With this abonimation of a tune, the car was rock stable in mid-speed corners - mega understeer, then oversteer on demand on throttle - but the moment I got into a high speed bend, there was that same behavior: the rear just let go as if a switch were flipped and all my tuning had been turned off.
I will do some more testing on Tokyo when I have the energy. But these initial findings are NOT encouraging.
FINAL THOUGHTS
OP is right in their criticisms. The Superbird is an undriveable joke because there's no option to add rear downforce. The Porsche Carrera RS 2.7 isn't quite as bad, but it is still terrible. This being a classic with an extensive racing heritage, I don't get why we don't even have the option (all I saw was cosmetic stuff in GT Auto) to add rear downforce and lean heavily into the old race car vibe.
My old A110 is a total mess, but with a silly rear-heavy downforce tune, it's lovely to drive now. This Porsche could be the same, but it isn't because... reasons (PD).