I went on a tangent about Rudy Wiedoeft assuming that he could be the alto sax player when it was clear that you had mentioned him only as a comparison. Incidentally, there is a short film where Rudy and Benny appeared together, talking to each other and fooling around with their instruments.
I listened to a lot of Jimmy's recordings and I go back (and agree with you) with my original view that the sax player exchanging phrases with Bing is unlikely to be Jimmy. I listened to several recordings by Benny Krueger and he is likely to be the saxophonist exchanging phrases with Bing in Star Dust.
I hope other forumites chime in.