Hi
I totally understand your post-purchase nerves. However, you have nothing to fear. I have used a jazz bass for the last 15 years and have never played slap on it. In fact, most of my gigs involve playing Country, a genre in which the P Bass is supposed to reign supreme (older country anyway). You will have no trouble getting a big bottom end from the jazz. In fact I feel it has more true low end than the P, but with less low mid. The only jazz bass tone I don't get on with is the trebly back pickup soloed sound, which seems to be the default tone for many jazz bass players out there. It does so much more than that. Try using both pickups together and then back off the back pickup by various degrees to see which sound you like.
The other thing to remember is that you liked the sound of the bass when you tried it. Trust your own ears!
David.