In my opinion it sounds like your clutch is slipping. Take the car to a flat surface and park it and pull up the e-brake. Shift the car into 4th gear I think it is. Then slowly let out the clutch just like you do from a dead start. If the car stalls then it isnt the clutch. If you are able to let the whole clutch out and the car still runs and kinda sputters like it wants to take off then it means your clutch is slipping real bad.
When you get on the gas hard, do you smell anything? If the clutch is slipping badly, you'll smell something like hot brakes.
I think a better test for a slipping clutch, is to get on the highway and put the car in 4th gear. Get the rpms close to where you make peak torque (3000-3500ish) and punch it. If the rpms rise quickly then you know the clutch is slipping. If they rise slow and steady as usual, then the clutch is holding. The most load you can put on the engine and the clutch is high higher gears at peak torque. This test will tell you without doubt. The E-brake test seems worthless to me. That only tells you if you have a COMPLETLY worthless clutch, as you will be no where near peak torque or loads.
It should normally engage near the top, so that is normal and doesn't tell you anything. You'll just have to try the 4th gear tricks listed so far. You may be noticing the warmer weather. My GT always feels like a pig in the summer and surprisingly quick in the winter.
Yeah Ive never done it before on a car but Ive always read about and stuff. People say its a good way to test for slippage since with the e-brake pulled up the car should stall. But if the clutch is slipping bad it will just kinda spin freely without transfering any power to the car before stalling out.