I think -18dBFS is far to low. Therewith we reach a point were the noise-floor becomes a to high influence. That, is even more true for non-linear dsp's, which may multiply the noise floor with produced harmonics. That is also the reason why most distortion/fuzz plugs in the digital domain filter out the high/low end of the spectrum. Mostly they filter against 200Hz - 5000Hz.
That wont be suitable for bass, for example, and, also wont make it for lead distortion.
In the digital domain, dBFS, means nothing. This becomes meaningful just when we enter/leave the barrier from/to digital/analog. So there is no reason not to use the full float range.
However, implement a selectable expected input level in the dsp/plugin structure is easy and already done for the GxPlugs, just, the implementation in the GUI is were I'm hang. I need a nice little 3-way-switch image to implement.