While looking for research on a slightly different subject, I came across this study, which shows a fairly significant amount of improvement in untrained individuals engaging in a chronic (long term) stretching program.
STR had significant average improvements (P < 0.05) for flexibility (18.1%), standing long jump (2.3%), vertical jump (6.7%), 20-m sprint (1.3%), knee flexion 1RM (15.3%), knee extension 1RM (32.4%), knee flexion endurance (30.4%) and knee extension endurance (28.5%). The control group showed no improvement.
However, it can be argued that for an untrained individual, any kind of muscle stress (Which stretching is), would produce improvements. I was unable to find any studies looking at performance improvements in already trained individuals. There are many that study the effects of acute (short term) static and PNF stretching before performance (Which has been shown to negatively affect power output), however.
Is there a study or other evidence that a long term stretching program provides performance benefits in a trained individual?