If you're going for reliability and power, stay with the Firebird. I owned a 2000 WS6 for over 70,000 miles. Unless it has been poorly maintained, the Audi will not hold a candle to it's reliability, lol. The maintenance requirements are also far less. Altogether cheaper to own. Fuel mileage will probably be similar as well. I consistently got 29 all highway with mine (6-Speed), 25-26 average combined 50/50. I also drove normally, and didn't beat on it all the time. I'm guessing your mileage is lower given the mods though.
Torque is also something that will not be the same. When I dyno'd my WS6, the torque was over 300 ft/lbs at the wheels at about 1800 where the dyno started reading. It held perfectly flat all the way to about 5,500, where it tapered off to about 250 at redline (6,200). You know what I'm talking about, you hit the accelerator, and you have instant power regardless of RPM, and it pulls hard the whole way.
If she wants performance, stay where you're at. It's cheaper, and far more powerful. You can't beat the LS platform. If you put what it'd cost you to get the TT to the same power level you're at in the Firebird in the Firebird, I'd bet you could hop over 600 HP to the wheels. I can't say enough for the LS series in regards to performance and reliability.
Now the TT will be something new. It'll also have more features and feel like a better built car. It'll feel higher shelf, and I'm certain it's fun and can be made fast. If you get the Quattro, it'll also be waaaay more practical for winter. I'm good in snow because I had to be to get anywhere in winter in that thing. The stock 275/40's aren't incredibly wide, but plenty wide enough to make that thing a sled on even an inch of snow.
To sum it up, I'd say this. If you're after a fast, inexpensive to own and reliable car, I'd keep the WS6. If it's more about the driving dynamic and user experience (comfort), and you don't mind more expense and maintenance, go for the TT. If you get significant snow, an additional nod to a Quattro TT.
If it's so great, why did I sell mine? lol. It was a hard decision. Felt like I betrayed the best car I have had. All I did was plugs, oil changes, and brakes the entire time I owned it. She never let me down. I sold it to fund the Audi project in the meantime though. Then I'll get back into a LS powered car in a few years, hopefully in Corvette form, lol.