About 1 tenth and 1 MPH.
Went to the track tonight with the new skinnies on the front. Bogart P-1s with 165r-15s. They weigh 25lbs each which is 17 lbs each less than the stock 16" GSCs. Only 34Lbs weight removal but with great effects. Last week in simular weather I ran 12.4xs @ 109. The baro was a little better this week but the temps and humidity were the same. My runs tonight ranged from 12.18 to 12.25 @ 111+mph. I figure that the baro accounts for probably half of the gains.
I'm definitely happy with the results of this mod