Why does my car feel slower? And why does the trany clunk?

Ok, so recently I have noticed that my 2001 celica gt just feels really sluggish. I remember when I first got it and I felt like it was actually a decently fast car. Now it's so sluggish that I think my girlfriend's 99 vw bug could beat me in a race. Why does it feel so slow? I have put 20,000 miles on this car and the mileage on it is 127,000. When I try to race, I notice that at a certain point, the power drops slightly as I am accelerating. I can't say when my car started feeling slow. But, I definitely notice it now. Also, my transmission clunks sometimes when I shift (especially if I try to race). What is going on? Can anyone help?

Background info:
- I just put a stock exedy clutch in it about 2,000 miles ago because the trany was starting to randomly make a small clunk noise after letting the clutch out (now that noise is much worse and more frequent)
- I checked my spark plugs and they look fine, but I did not check the gaps
-The Air Filter seems like it is in good condition
-I average 32 mpg on every fill up and my commute is 30 miles every day which are mostly on the freeway
-I have two small oil leaks. A timing chain cover gasket leak and a valve cover gasket leak. These are not significant enough that I need to top off my oil between oil changes though. I could make it at least 10,000 miles before it hits the low line, but I change my oil every 3,500 miles.
-And I did the butterfly valve removal mod (afterwards the car idled lower and has a small knock only when idling)