I have a Rhino .660 tube on my SBE2 and I have been shooting the Nitro 4x5x7 3 1/2 inch 2 oz. loads for three years now and I swear by it.
I will not hesitate a 60 yard shot, and I have made several. My furthest kill was 84 with it (double tap).
A lot of people have put a lot of money and effort into building the ultimate turkey gun on the foundation SBE2. Since you already have the foundaion, go ahead and complete the structure, you'll be happy. I remember that I originally didn;t want tospend the extra money on the tube and the shells, but I am glad I did.
Definitley put a sighting device on the gun. At 30 yards you have little room for error with the tight choke, and I have found the Nitro shells hold tighter than others as well.
Just one more note...I shot a bowling pin Saturday at 100 yards, and there were 18 pellets in it....deep.