Got to say that from the first the JTS felt like it had definitely got more poke to it.
I had to hand in a GTV 2ltr TS and exchange it for a 156 JTS (needed the space). You can tell the difference straightaway. The power comes on earlier and there seems to be more of it. There's none of that hesitation you can get with the GTV where either nothing happens for a second or two or it wheel spins. the 156 JTS feels a lot more responsive on the throttle.
I always found the 2 ltr GTV to be underpowered for its looks, an extra 15bhp would not have gone amiss.
Wait for the JTS, better still get the V6. Then again, aren't they going to put the GTA engine in it as well? Now that would be worth waiting and saving your pennies for.