I appreciate all the replies, but this is what I'm really trying to understand, whether a mastering plugin can raise levels sufficiently without any use of compression.
Define "sufficiently" precisely, including target loudness, peak max and dynamic range and include the input audio characteristics, i.e., those same numbers (loudness, peak and dynamic range). Then, the answer is a simple yes or no. (This doesn't mean the "mastered" mix will sound good, though!)
Simple example, if your mix currently measures -25dB (LUFS), and has peaks of -5dBFS, and you want to get to -12dB (LUFS) loudness, you cannot do that with simple gain because it would push the mastered peaks +9dB to +4dBFS, creating clipping in the master when bounced down to (e.g.) CD format.
But, there are so many variables you cannot say how it might be done. Limiting can fix the peaks, but it's really a kind of compression, and pretty drastic if that's the only tool you use, assuming the peaks are really part of the music, like snare hits. But, maybe the peak is just subsonic rumble you can't hear on your 3" monitors, and a simple HPF knocks 6dB off that, then problem gone. Or, maybe it's only a half dozen snare hits and you can automate those down. Again, gain will be adequate. But, if it's a hundred peaks, and they are important, well, you have some work to do (and probably it's not the mastering step where you want to start addressing that, IMHO).
Your question suggests that you have not really dug into all of this stuff as much as necessary to decide whether you can let something like Ozone make decisions for you. Someone else can't decide without at least knowing where your mix is at, and where it needs to go, and, maybe even what "mastering plugin" you are talking about.
p.s. (edit) And, of course, mastering is about much more than the level, and that part of it is after whatever tweaks are done to EQ and balance, or any other subtleties that may impact loudness/peak adjustments/corrections done subsequently. (Dynamic EQ has its own compression impacts, being a very finely tailored kind of multi-band compression, and that's probably lurking in your mastering plugin.)