Can't claim I've ever tried it, but my $0.02:
First, both the diff amp and the scope will have a roll-off. The frequency response of the amplifier will be an issue anyhow, so this is only useful if the scope is the limiting factor. Let's assume this is the case for argument's sake. Let's also assume that the scope is bandwidth limited, not slew rate limited.
The differential amplifier in comparison mode will add/subtract a DC offset to the signal. The spectrum for the signal before and after the amplifier will look exactly the same (assuming an ideal diff amp), except for the magnitude of the peak at DC. The roll-off of the scope will only affect the high frequencies, so a 50 MHz sine on a 50 MHz scope will still have its amplitude reduced by - 3 dB. If we split the signal from the diff amp in a DC part and an AC (50 MHz sine) part, the DC part will not be affected and the AC part will be attenuated. Changing the DC part will do nothing to change the roll-off of the AC part.
Now if the scope is slew-rate limited, especially an analog scope, then using only the center graticules may help slightly. You don't need a differential amp for that though: the vertical attenuation and position control will do just fine.