Alright... I'll help you out, but you pissed me off by calling me dumbass.
I've never worked on the first gen GTs, but if they're anything like the 2nd gens, the hoses leading from the intake hose go to the idle air control valve. It's en electrically controlled valve that bypasses the throttle plate so your car can hold an idle. Messing around with this will do nothing for your backpressure.
Backpressure is sortof a misnomer. Ideallly, there should be no backpressure in your exhaust... the point of your exhaust is to coordinate the pulses of gas flowing down the exhaust with other cylinders so it scavenges gas out of the other cylinder. The reason why cars want "backpressure" is because they use manifolds and not headers. With an exhaust header, you split each exhaust line up and run them independently for a certain lengh, x, which is a function of your exhaust gas volume. With a manifold, this idea doesn't exist because basically you're just dumping all the cylinders into a pipe and the whole thing is a freakin mess. Anyway, you need some type of pressure in the exhaust pipe prior to that or else your performance goes to crap.
Depending on how your valve timing is, it also helps bring in the fuel/air mix for the next fire of the pistons. Really, this concept of backpressure has nothing to do with pressure and everything to do with vacuum. Any neutonian fluid will decrease in pressure as the mean stream velocity of that fluid increases. This is why planes can stay in the air and why the backside of a spoon will be attracted to the water flowing from a faucet. Anyway...
The reason why you don't want an exhaust that is too large on your engine is because if the volume of the exhaust is too large compared to the exhaust volume of one cylinder, you won't get this scavenging into the other cylinders.
With that said, chances are that the turbo muffler you put on and the cat you put on increased the stream velocity so much that the velocity of the exhaust gas has died PRIOR to the next cylinder firing. One of the reasons why some cars have resonators is because it artifically slows the flow of gas down and helps create some type of scavenging. Because of this, the reason why you're getting the extra feel of oomph in the higher rpms is because if you assume that the time required for the gas to travel down the exhaust is fairly constant, than the number of "exhausts" of the cylinders per unit time increases as your rpms increase. Basically what I;m saying is that you're experiencing the scavenging at higher rpms because you're essentially getting rid of the exhaust in your pipes faster than you should by killing the cat and the muffler.
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