Sorry I might be wrong but from what I was told the n54 is a powerful engine and can withstand 700hp on stock internals no problem.
Just because it 'can' do it doesn't mean it's not 'on edge'.
At 700whp, you will likely start to face issues you don't normally see at 600 and below. For instance, you may at that point need a better/enhanced ignition over stock coils - be it bimmerlife or Precision Raceworks. You also start to see increasingly common spun hubs at that power level, requiring remediation of varying methods and degrees of success.
With no E85, you're gonna need a lot of meth, which will need a controller, likely a direct port setup, which adds some complexity, regardless of how little, its still more complex than crank up the car and go. Yes there are protection methods available, but nothing is foolproof, so there is added risk.
If your tuner recommended you go with Pures for 700whp on stock internals "no problem" with no E85, I would find another tuner. I cannot think of one well respected (hell, even disrespected tuner!) that would suggest that setup.