got the big bad XR7 back today

After a few months on the shelf, the supercharged XR7 came back this morning, after replacing a lot of the rear suspension, but its finally back... its an automatic, and I got a few questions pertaining to racing and accelerating and stuff... I was driving down this road in town, the speed limit is 45, I was doing a little more than 50, so this guy in like a 97 Altima tries to pass me, so of course, being the dickhead I am, I floored it and didn't let him pass me, he tried to still pass me, but couldn't for obvious power reasons... but it was weird like I could hear his engine redlining going nuts, but I had it floored and I was doing about 2000-2500 rpms (I redline at 5000), so then after I was thinking, since the max horsepower is listed at 2600, what if when I was racing I put it down into "1", then shifted to "D" after the rpms started getting up there... so I experimented with this, but once I get above 3000 rpms, the engine feels like it is working so hard, but not really doing much more, there was a slight difference, a little quicker... but is this bad for the transmission, because this car is 12 years old and if I blow the trans, the car will pretty much get junked, also has anybody experimented with manual shifting their automatic cars, I am wondering how some of you guys feel about the difference between that and just letting the car shift itself...