It doesn't make that much difference in lap times aye... If you are a good racer and can put down consistent fast laps, there won't be any much wheel spin involved. The car will feel very different though. When i first wacked in my cusco 2way i was expecting it to give me massive advantage... it just wasn't there, sure the car felt much nicer to slide around on and off power... but laptimes are pretty much the same.
The biggest difference i felt between 1.5 and 2 way was when you car is already sliding, and you lift off the gas, with 2 way...car keeps sliding... with 1.5 way.. the car will try to get grip again...stopping the slide.
hope that helps...~!