Really ancient forum now, but with insight.
Recently I got a friend to agree to taking turns karting with 20 pounds of ballast. ( 4, 5 lb. weights in our pockets)
I weigh about 220 pounds, my friend about 180 pounds, so the ballast represents half the difference.
We each took our assigned Karts through 4, 12 lap sessions, two with added ballast.
I compared a weighted average (ignoring the slowest 4 laps) of lap times for each session. I also compensated for the warmer tires and other factors that affected times between sessions by finding the average lap improvement from one session to the next for the non ballasted drivers.
Based on our (not yet as large a sample as i'd really like) data, in a ~30" lap 12 turn lap, the penalty worked out to .03"/lb., drving a Sodi RT8 Kart
It was at TBC in Richmond if that means anything to anyone.
As far as the other factors that affect lap times, there are many, and sometimes, it is the kart that makes the difference. On previous sessions, at the same track, we've swapped karts between sessions and swapped results, from my friend besting me by 4/10ths to me beating him by 2/10ths, we both set our personal bests for that track in that Kart.
I'll post more when I've got a larger collection of data, I want to get more drivers in on the deal.