I'm the guy that started the thread over on the TAS website that M60gunner mentioned. Back in March this year when I first got my Wesley Special, I took off the B50 string it came with and replaced it with a 8 strand 450+ string padded to 12 in the loops. It shot well, but the bow was not a quiet as I thought it should be and to me it just felt like that string was too harsh on the bow. No scientific data to back that up, just felt that way to me. Here is a link to a youtube vid I did shooting it with that 450+ string. Arrow was 535 grains. Pay attention to how the string wobbles after the shot.
I decided to put the B50 string she came with back on and try that. To me, I perceived a big difference. The bow seemed quieter and more well behaved on the shot. With a 535 grain arrow, still seemed pretty quick to me. I emailed the bowyer, Craig Ekin, to ask him about the string that came with the bow. He said that if the customer does not specify otherwise, he send every bow out of his shop with B50. That was good enough for me. Here is a link to a youtube vid of me shooting that same bow with a 535 grain arrow with a 12 strand B50 string. As before, watch the string after the shot.
Only you can decide what is best for you. Perhaps get a Fast Flight string and try it. Many variables and it might work out for you. But for me on this bow, I'm sticking with Dacron B50 or B55. Also, don't worry too much about stretch. If the Dacron string is built right, it should not stretch on you much. I build the 12 strand B50 you see in the video and when I first string it, it braces at 6 & 1/4" and after shooting a couple arrows it comes down perfectly to 6 & 1/8 and does not move after that. I make the splices 7.5" long. Best of luck to you!
Steve