I really dont know much about the Nacra 5.0 set up... but unless the current jib blocks are on the front beam also i am sure it will effect the shape of the jib. As to how it will effect performance..... unk (but my guess is it will change the shape/effect the pocket/ reduce performance)...

I would try the $3.00 bungie solution first to see if this handles the issue and leave the boat stock for a while.

Do you have a picture of how the jib blocks are on the 5.0?