Make sure you drive a few if you're going for a petrol. So many of the variable valve timing cylinder things are fucked you wont realise until you drive a good one and it's got twice as much torque.
No, it helps mid range torque. If you can drive both it's easy to feel it, but a broken one doesn't feel all that broken in isolation.
They do tend to make a diesely rattle when they start if it's kippered, something about the oil not pressurising properly. Working ones start silently.
Make sure you drive a few if you're going for a petrol. So many of the variable valve timing cylinder things are fucked you wont realise until you drive a good one and it's got twice as much torque.