Dropping the index would reduce the amount of i/o that had to be done and so improve performance, but I don't think it would help much. I don't think the insert itself is what is slowing things down.
In general, are you really sure you can't rewrite the proc?
If you can't change the proc, you are left with trying to make it run more efficiently. So look at performance and turning on the queries in the proc. Are they using indexes? Are statistics up to date? Would an index on a different set of columns be better?
-bret