a level difference of 5 is HUGE in terms of in game combat.
No, it isn't. If this was true, then FPs would not be rated for ranges that are up to 6 levels wide. Also, the range at which you can be auto-grouped into them is even wider, because it will let you in a little lower, and will still group you into it when you're a couple levels higher. The time and effort required to complete the content is substantially the same regardless of the level of players in the group. You might have some chance of dying if you screw up at the lowest levels, and no chance of dying at all on the highest, but that's about it.
To the extent that level matters more at lower levels than at high levels, this is already accounted for by the point at which things go grey to you at each level (it happens with smaller gaps at lower levels than it does at higher levels). If the level of the lowest level person in the group would represent green content for the highest level person, then there is no reason for nerfing the lowest level's XP. If the lowest level would be grey for the highest level member, then nobody should get any XP from the group.