Sorry, I think I have confused things with the 50mm comment.
Most of the time I will be about 10m from the stage and shooting in the 28-35mm range. So please ignore the 50mm range for now!
Good to know that you can see difference in Lens sharpness at ISO 800.
I will have a hunt around my local camera shops in the UK and see what they have, and if they have any of the lenses you suggest second hand.