Jumping from Forza 4 to Forza 6 was/is definitely a bit of a shock from the tuning stand point. Cars are much more touchy and cagey in 6 than in 4. I wasn’t high up like the poster, but I was in 700 range on the ldrbrds myself. Initially I tried just going with my usual tuning style which of course did not go over well in 6. Spent a few days in frustration trying to work out a new tune. Checked out Walkers guide and one thing I tried was to not even install sway bars and use no toe in front or back. BAM! Using my tune style, removing the sways and using no toe finally gave me a much more smoother drift. It’s still nothing like the smooth and stable that I had in 4, but now I can actually jump in a lobby and not be one of those kids spinning out everywhere!! LOL
Someone else noted above about throttle control… This definitely plays a big part now. I was already in the habit of throttle control due to my tune style, but noticed it was needed much more in Forza 6. Learn to feather your gas finger on that trigger now, you can’t just hold that trigger in going around corner.
Torque also is far more touchy in Forza 6 as well as the turbo lag. I’ve taken to using super chargers now when they’re available just to cut back on potential over powering mid turn.
If you do have a car that’s got a lot of torque, you might consider trying a wider rear tire width until you get control of the rear end. Careful on lowering your accel. You might think it will help but what will happen mostly is if you go too low your car will simply straighten out in a turn or you’ll lose the spin on the tires. You might consider upping the decel instead and going with wider rear tires.
All in all, Drifting in Forza 6 is far more work and becomes a bit tiring rather quickly. I used to drift all day in Forza 4. I can only deal with drifting in Forza 6 for a few hours. It’s not just the whole tune situation either… I’m getting so freakin tired of people just crossing the line without even thinking of waiting for everyone else. I’m tired of getting t-boned in turns because kids come in way too hot and don’t have enough angle. I’m tired of kids who can’t control their pace and instead of backing off a bit, they just keep coming until they run into you. The timer is 29 seconds for all tracks, small or big. There’s too many laps on all tracks. You can’t mute fools during the race. You show up to a lobby, see some kid in an X class, you can’t vote kick him, that option is not available it seems until you’ve raced with them. I don’t even know if the vote kick system works because there’s no indication if someone is up for vote kick or not. Then there’s the lag! Never have I had this much lag in Forza 4…
I digress, this was about tuning, not complaining… LOL… sorry, old guys, we tend to ramble. 
In the end, I think the tuning is simply more touchy and more time will be required on each individual car to get the tune right. I’m sure that one can get a smooth and stable drift out of a car in Forza 6 to where drifting won’t be as much of a chore, but it’s simply going to take more time fine tuning to get it right.