Can you actually decrease the C.I.D. of a racing engine to increase performance?

May be...

Will NASCAR allow you to undersize on the specifications to let you "play" with performance modifications?

yes you can, and it will happen somewhere in the future

Yes, size does not always matter. Sorry ladies......

About 6 years ago, Jimmie Johnson was getting unbelievable gas mileage. He was literally getting an extra 10 laps more than anyone else.

Nascar asked them what they were doing. Chad answered that they were doing nothing outside the rules. Nascar then told them to disclose their "secret" or face a suspension.

NASCAR had to eat crow when Chad announced what they had done was put the next size SMALLER carburetor on the race car. It meant that they gave up 2 hp in the track runs, but got that extra 10 laps in fuel mileage.

NASCAR will let you play with the numbers to a degree..
Any major change needs NASCAR approval..
That was a neat trick by Chad using the Holley 740, until the other teams caught on..

You can not decrease reciprocating mass. Nascar has minimum weights for rods, pistons, crankshafts etc.

Decreasing the displacement is allowed but gives no advantage because the rpm is limited as well as the maximum bore size.air vouchers