For your car, I don't really think it'll be worth it. In fact, it may do worse (I've heard of this happening b4). Probably the only real way is to get the car dynoed with shorties, then dyno again with long tubes.
Long tubes are not needed (in MOST) cases until you got major air flow through your motor. Long tubes are usually reserved for and give the best hp/torque increase when with blower motors or high compression motors that are running ~350+ horses. I've learned that bigger is not always better (eg, jumping to a 4.10 gear instead of 3.73s, b/c it sounds bigger, doesn't always work out that way). There are a number of variables you have to take into account. Hey, i'm not saying it WON'T work, what I'm saying is that if it doesn't hurt your hp/torque #s, it'll only help a tad, probably not enough compared to the $$, time and labor.