Yeah I'm surprised it performs as well as it does under so much stress to be honest. Especially when you play F1 2015 which, although a great racing game in it's own right, suffers quite badly from slowdown on specific tracks when the rain sets in, and that's nowhere near Pcars physics.
As for the silly notion that "[X game] isn't a sim like [Y game]", that is just a stupid argument that little boys have over their toys. Forza, like GT, is by definition a sim.
People have made up this imaginary genre called "racing sims", where only PC games derived from ISIMotor are regarded as "true sims", and everything else is "simcade". In this world, words like "arcade" are dirty words used to put games down.
A racing simulator, is simply a racing game designed to simulate how a real car behaves, and even one which simulates a car's behaviour poorly is still a bloody simulator. Even the first GT game was a sim, and it's physics wouldn't have been a 10th as complex as Driveclub's, and Driveclub was consistently put down by sim snobs for being "arcade".
You can argue what each sim does well compared to other sims, and which ones don't do certain things so well, but flat out calling some of these games sims, and claiming others aren't sims, based solely on your belief of which ones are more realistic, is just stupid.
You see, the problem is people are trying to shoehorn these games into discrete labels. The spectrum from arcade to sim is continuous, like a rank from 0-100%. You can subdivide the line into "categories", but even in the same category there will still be games that are more sim than others. Then there's the improvements in computing power from one generation to the next. What was a sim back in 1998 would be simcade right now. Also different people have different driving styles, different expectations of reality, different skill levels and different controllers (keyboard/joystick/wheel) set up different ways and you muddle up the comparison further. No one's gonna argue that NFS is arcade and AC is sim, but comparing which sim is better is not very straightforward.
For me, I divide the spectrum into 7 categories:
Fantasy - WipeOut, Mario Kart, Modnation, Trackmania
Arcade - Outrun, Burnout, Blur, Split/Second, Sega Rally, Ridge Racer
Semi Arcade - NFS, Driveclub, The Crew
Simcade - GRID, DIRT (except Dirt Rally), Codemasters F1 series, NFS Shift, Milestone WRC series, SLRE, GT1/2
Semi Sim - GT3-6, Forza series (excluding Horizon), Ferrari/Supercar Challenge
Simulation - GPL, rFactor 1/2, GTL/GTR2/Race series, LFS, iRacing, Enthusia, PCARS, AC, Raceroom, Automobilista, RBR, Dirt Rally
Real Life - self explanatory
Full grade industrial simulators used by professional race teams in F1 are probably a step above Simulation, but I doubt it's 1:1 with real life yet. Otherwise teams won't bother with real life testing (more expensive, more hassle, more danger, etc).
I'll also add that at least 90% of the "true sims" people refer to when putting down Forza and GT games, are based on the exact same 11 year old game engine. At least PD and T10 write their own engines.
Age and using someone else's engine doesn't make it any less valid. ISImotor is over a decade old and has been used by more than a dozen games but it was way ahead of its time, and it certainly feels more realistic than GT6 still. If you look at Automobilista (basically ISImotor Perfect Version) it can still trade punches with AC on a few aspects of the simulation. It does have limitations though, and has less scope to expand compared to the versatile engine in AC for example.
Sega Rally is an arcade game, Daytona USA, and even the more realistic sim level Ferrari F355 Challenge ( its coin op ) People would misunderstood that though, how times have changed 
