Things you did/upgraded that surprised you how helpful they were

Replaced a stock tank that had been sumped, with a fuel cell. Must have fixed a delivery problem too, in the process.
Picked up almost a second in the quarter.
Also changed the 4200 converter (different car) when it blew up, to a munsinger 8" that stalled 5000. Picked up a couple tenths, no huge surprise, but picked up 3-4 mph in the eighth too.