I was thinking the same when i had my R33 Skyline. Was thinking of doing it up or getting a new car. Started doing it up and spending money, but I was still thinking of a new car.
In the end, if your thinking of selling it and getting something new, you wont be fully happy until you do. The respray might keep ya happy for a while, but youll still be thinking of a new WRX. Imo save the cash that you would spend on a respray and use it towards the new WRX.