So what if your friend, in full level 70 epic (PvP Gear or Sunwell Gear) decided to level to 74th.
That makes the raids to get the level 70 gear much easier, due to smaller threat ranges and larger damage from attacks.
It makes you four levels higher than all of the level 70's that you are competing against in your bracket, which is a substantial advantage in pvp.

And with your raid rewards and/or pvp honor you can still buy the level 70 gear and equip it.

How is a level 74 not stronger than a level 70, when the bracket is 70-74 and both have access to all of the gear that a 70 can use, but the 74 can access some level 71-74 items that the 70 cannot equip?

I would definitely consider the exp turned off.
But at x9th or x4th, in whatever bracket.