I think a well setup suspension will be the most significant mod in terms of improving the potential of the car besides maybe tires. There are so many variables.
Notice I said well setup. I've seen an E92 M3 on motons struggle to break 2:00 at Texas World Speedway. I've taken my s/c'd 330 on STREET tires and k-mart coilovers compared to the motons on the same course and direction...did a 1:57:32.
Will I say that my 330 is superior? No...but the driver and the SETUP was!