I agree with Matt that in general distortion performance and sonics will tend to not be as good in bridged mode as in stereo mode.
The reason that 4 ohm bridged mode power ratings are not specified for most bridgeable amps is that in bridged mode the output stages of the amp will "see" a load impedance equal to 1/2 of the speaker impedance, at any given frequency. So a 4 ohm speaker would be "seen" by the output stages of the amp as if it were a 2 ohm load, which is too low for many amps to handle with good results, or even at all. And the impedance of many "4 ohm" speakers will often be considerably less than 4 ohms at some frequencies (although that is not generally true in the case of Maggies). So in those cases, at those frequencies, the amp may see a load equivalent to 1.5 ohms or even less.
That halving of the effective load impedance is a major reason for the degradation in distortion performance that tends to occur in bridged mode.
The reason that halving occurs is that in bridged mode equal and opposite voltages are applied to the two speaker terminals, rather than one terminal being grounded (i.e., at zero volts). So from the perspective of the output circuit that drives one output terminal of the amp, a given output voltage will result in twice as much current being drawn as in stereo mode. Per Ohm's Law, that doubled amount of current is the same as what would be drawn from that terminal in stereo mode, for the same output voltage, by a halved load impedance.
Regarding the question of how much power the Maggies can safely handle, I have no idea.