It would be interesting to test that theory, i think the advantage you'd get off the line due to less weight would more than make up for the acceleration you lose through the extra drag at higher speeds.
But for me the amount of effort required to remove then replace the doors/bonnet is not worth the reduced time for the run.
Wash the crud from under the wheel bays, up to 2 kg saving there!
Empty the door pockets and ash trays.
Fit slimline spares to the rear. 10kg each!
Hoover the interior.
Don't lighten the front or wheel-spin may become an issue...
On a slightly more serious note, you can change the car battery for a Gel battery (save 8KG), light weight alloys - OZ have just brought out some new ones - 17" rims @ 6KG each - nearly half the weight of Alfa rims