Recently I ordered, from my bow's maker, a second set of "fast-flight" strings, identical to those I purchased from him a couple of seasons ago.  That set worked well with 200-210 grains up front.
When the new strings arrived, I set them up the same, although with musk-ox wool silencers instead of sheep wool.   Shot the same arrows, and they consistently shot as under-spined.  I had to back off to 125 grains to obtain good flight.
The new silencers were advertised to be lighter than the originals, but I did not think the difference was enough to explain the results. So I called the bowyer, and he allowed that he had substituted a spectra-fiber string material, as he was unable to get the kevlar material anymore, and that he had not been aware that it would make a difference.  
End result for me---new arrow shafts --heavier, tougher, quieter, with better FOC--and a big VISA balance.

Just thought folks should be aware.

Bruce,

I suspect you are talking the difference between Fast Flight and Fast Flight Plus.  I'm not sure that Fast Flight is even available, unless someone has some old stock sitting on the shelf.

Bill