For the parameters with a linear range you have to scale the the parameter range to the raw value range.
In your first example -5db would be 0 and +5db would be 16383.
The Request String Parameter SYSEX function hasn't been updated since the reverb update (at the very least) and can't be used for every parameter.
Some of the reverb parameters, like the Decay Time, have non linear ramps in the range that can't be queried. I've opted to interpolate between fixed points for my own use but that is a bit tedious if you want to map every parameter.