A problem with power after transmission rebuild.

I bought a mustang about 3 weeks ago and it had a bad transmission. Today I got it out of the shop with a new trans and it feels like it has no power. When you hit the gas (floor it) the engine revs and it feels like its spinning and power gradually comes in. However when I observed my father driving it, i noticed its not spinning at all, the engine just revs high. its like its slipping or something. Does anyone know what might be wrong or what direction I should go with this. I just want to feel all 300 lbs of torque the car has. I have an 06 sonata and it pulls alot harder off the line (and the wheels spin a lot ) with its 235hp and 226 torque @ 3500rpm. Please advise, I've dumped a lot of money into it so far and I just want it to perform the way it should. I have no idea what the problem might be so any help would be GREATLY appreciated.

Power problems

Thanks for the input fellas. It's being taken back to the shop Monday morning so they can take a look at it. It also is quick shifting from second to third when you manually shift it from first to drive, so that's something else that has to be looked at as well.

update

It appears to be the speed sensor for the transmission. I spoke with a friend whos had mustangs all his life and I told him that my speedo in my car is about 20mph over what its really riding at. He said instantly "It sounds like you have 4.10 gears in the car". I told him about it quick shifting as well but we didn't have hardly any real time to talk (we're both insanely busy with work etc). Conversation quickly changed to his mustang (which I may be purchasing soon w/ 500+ hp [without the 150 shot of NOS]). I'm wondering what steps need to be taken if you change the gears in your car (I never got to ask that question) so that it runs properly and the speedo reads correctly? What I get from this is that the gears were changed (i don't know much about the car at all nor does the previous owner) and that made the speedo go out of wack, which in turn (due to the speed sensor) makes the car shift early since the tranny thinks the car is going faster than it is, its shifting early. Any thoughts on any of this? Has anyone else experienced a problem like this?