The LP-64 is a Linear Phase processor and should have less coloration than the Sonitus, and therefore to me is more useful for mastering purposes. However I don't find the Sonitus to have all that much color to begin with. Either way, the linear phase characteristics of the LP-64 should make its employment less noticeable when used judiciously on a master mix.
While true that Multiband compressors are often used to fix a mix where things could perhaps be corrected prior to mastering, IMO they have their legitimate uses in both mastering and even in the mix process. Now you're likely not going to use a MB compressor on an individual track but they can be quite useful on groups of tracks or buses (drums, keys, etc).
MB compressors can sometimes do wonders on things like drum buses or full range loops with inherent problems or unwanted characteristics. In a way, you can use the MB comp as a kind of dynamic EQ processor. Basically you can shape the overall tone of a bus (or mix) but in a way rather different than using a traditional EQ in that its effects can be tailored to respond to only certain kinds and level of input - in the same way a compressor in general can be tailored to respond to input.
Anyway, I'd say play around with it. Obviously if you are "stuck" with something delivered that doesn't sound quite right to you - but also if you are having trouble getting a group of instruments to have the tonal "shape" you want.
As was mentioned, don't track with a linear phase plugin enabled because they introduce lots of latency, but in the purely mixing or mastering phase they are perfectly usable.
Regarding Boost 11, and limiters in general: IMO they should be used very judiciously and sparingly. My own rule of thumb is that if I'm having to shave off more than, say, 3db of the louder peaks to get the desired volume level, then I need to go back to the mix itself (if possible) and work on basic EQ and compression elements. Limiters are a dangerous thing. Like salt in cooking - they can seem great at first, but are liable to squeeze all the dynamics out of a mix and make the whole thing fatiguing to listen to.
But a
little limiting never hurt anyone IMHO and is often necessary to get the desired loudness of an overall mix (and sometimes of a particular bus). Good thing is that the less the limiter has to work, the less any ill effects it has on the sound will be heard and/or noticed.
As I sometimes say, "Make music, not sausages".