Taking a few discrete points off rolling road data gives:
rpm, torque (base), power (base), torque (remap), power (remap)
1500, 100, 29, 100, 29
2000, 160, 61, 200, 76
2500, 160, 76, 190, 90
3000, 130, 74, 180, 103
3500, 110, 73, 160, 107
4000, 110, 69, 140, 107
So at 60mph at 2200mph there is maximum of ~160 lbft and ~70bhp.
Say 8bhp is being used to hold that constant speed, which leaves ~60bhp in reserve for accelerating or going up a hill.
By changing the 5th gear ratio and reducing the revs to 2000rpm to hold 60mph the reserve changes to ~50bhp, or a 15% reduction.
But if the a new 5th gear is used in conjunction with a remap then the reserve becomes 68bhp, or a 10% increase over the base vehicle.
I think all of this has taught me that a new 5th gear would need to be combined with a remap to offer the same in-gear flexibility as before.