Since my wife will use this car sometimes, the upgrades will have to be a secret at first.
In that case, look into one of the engine "Tune" products (such as by
Vermont Tuner and
Trifecta) which boost turbo-pressure and ignition that can be "switched" OFF by the driver. But be aware that both really need
higher 93+ octane fuel because of higher operating CR resulting from the increased boost pressure and advanced ignition timing. Not the
cheapest upgrade but certainly the
stealthiest upgrade, by far.
Oh, and don't FORGET to switch back to "normal" tune or your wife might accidentally "lay rubber" in the driveway as she backs out of the garage (ha,ha)!