I believe the recommended approach to the notorious "track day/domestic finance committee paradox" is to buy big, buy early and buy covertly. Think of the Iran-Contra scandal...get caught after you have completed your mission.
There are three possible outcomes:
1) Your are in Utopia (most unlikely).Your wife sees how much fun you had at the track and does not mind the expense because she will do anything to make you happy.
2) You are in a world of hurt. During this time you will realize that, on balance, buying gear and doing track days was totally worth whatever the consequences
3) You are in a world of poverty. The wife decides she needs those shoes, coats, dresses etc she has always wanted. You, however, have some cool wheels, great tires and awesome brake pads at your disposal for the next time/ next pay check. This will also be worth it
Basically, you cannot lose.