That's what happened to mine. Back in 2001, I was stage 4/5 running stock-like times in the summer. Turned out it was my clutch (which finally croaked that fall). In the spring of 2002 I got my JWT clutch (and my pre-cats removed) and my ET's dropped ~0.3-0.5 seconds and my trapspeed went up ~ 5-6 mph.
Z-Jay Edmonton Z Car Club 93 300ZX TT - Stage VI 01 Sentra SE - daily driver