I noticed that eight of the current top ten Lamborghini drivers are using Active Steering on Strong. I have always thought this setting to make cars understeer more. Four wheel drive cars understeer enough already in the game, so seems counter intuitive to me. Am I missing out on faster lap times by not using Active Steering?
Edit: Answering my own question with some testing. Grab your favorite drift tune, turn Active Steering to Strong and move rapidly up the rankings. I am working with a LP 640-4 Superveloce '09 and doing every tuning trick that I know to make the car loose. I watched the replay of the current leader 1:29.851 and he barely touches the brakes. Though his lines are super smoothe, the tires smoke on most corners. Will post my tune with Active Steering as soon as I get bored with chasing my ghost around.
I think you are probably missing out.
Some of the 'Super Aliens' are fast without it (Dan Holland and Wes didn't use it on the C7 TT at the 'Ring, for example), but most people use it - even Amo, who seems to be the fastest regular runner on Seasonal TT's.
IME, AS specifically helps manage the rear of the car, but it doesn't
add understeer. What it does is correct a lot of the tiny slides that are difficult for 'normal' drivers to react to quickly enough to to handle themselves... you can feel this through the wheel moving to apply tiny amounts of corrective lock as the car slides about. It also feels to smooth out throttle inputs on corner exits.
I use it most of the time as it allows me to make a car more unstable on corner entries (more rear ballast, less wing, higher rear brake balance, low LSD decel, etc), which in turn helps reduce understeer... which is the killer of fast lap times. I always want a car as unstable as I can reasonably control when it comes to TTing... the more skilled a driver you are, the more unstable a car you can manage.
I immediately didn't like the Murci, so didn't spent much time on it (c.30laps), but here's what I found...
The Murci is an understeering bag of rubbish (as are most 4WD and FWD cars in GT5), and this doesn't improve much with tuning. You can get rid of the initial corner entry understeer by adding rear ballast and/or running no/low rear wing settings. This will create plenty of initial rotation through momentum oversteer, but the more rotation you add, the more difficult it is to get through the fast turns at the beginning of the lap.
What I found impossible to 'fix' (similar to most 4WD cars) is the mid corner understeer... you can get the car to oversteer initially on entry, but as this bleeds away the car immediately slips back in to severe understeer unless your corner speed is perfect. Even if your corner speed is perfect, it immediately understeers if you pick up the throttle mid corner. And no amount of fiddling with toe/camber/LSD settings seems to cure it.
It's why I almost never bother with 4WD cars... they are just rubbish to drive.
I'm surprised by your AS/drift comment... I'm not great at drifting (I can usually get around 75-80% of the top score), but I always turn AS off to drift, and my scores improve as a result