I had always felt that my car was sluggish at the low rpms and I had to push the accelerator a long way before it did anything. Friends who had driven my car had noticed it too, they drive crappy old 4 cylinder Mazda's that are more responsive than my ZT180 KV6. I drove a mates Toyota Camry yesterday and the accelerator was 10x more sensitive than mine. After stripping down the VIS actuators and finding cracked solder joints and a burnt out PCB track I thought I was about to be driving a whole new car once I installed it again but it felt exactly the same. My car never had trouble at the high rpm end, kickdowns worked perfect and it flies after 3000rpm, it was just general soft driving that sucked.
I then figured out how to tighten the throttle cable. The cable is on the right side of the engine near the throttle body of course, and goes through a threaded metal tube which is screwed into a black plastic square plate that is pushed into a housing. You can pull this black plate upwards and out along with the cable and threaded tube and spin it around so it moves closer to the actual throttle lever, this effectively shortens the cable and reduces the slack. I turned mine about 6 turns, and it can probably go further but it's pretty dark out now I'll tweak it a bit more tomorrow. Took it for a test drive and finally found the improved throttle sensitivity I was looking for, it no longer feels like a wet sponge, and I'm confident it can be even better.
I highly recommend doing this if your car is old, it could just be a simple cable adjustment and not a $1000 manifold replacement. Just don't tighten it too much as it needs to return back to neutral position for the engine to idle normally. Be careful, especially if you're in an automatic, you don't want it to take off hard as soon as you put it in D!
edit: moved the pics up here
Heres the throttle cable..
Pop it out and spin the black square so it moves closer towards you.
Put it back and you're done, just make sure it still has some slack so it can idle properly.
The black square on mine was about halfway up the threads (before I took the pictures), you can see how far forward it is now, quite a big difference.