I doubt it would be any faster since this is a scripting language... each command is evaluated as it is executed. The only thing it would improve is by giving the scripter less ways to screw up his script (such as by forgetting to increment his variable or setting it initially). If anything, it would likely be SLOWER for having to ignore the initialize setting each time around after the first. Logically, a FOR loop is no different than a WHILE loop in this manner. It still has to execute var %x = 1, still has to check the condition each time around, and still has to inc %x 1 each time. Except for white space, scripting errors and the FOR loop having to ignore the initializer for each additional loop, they are no different.
