Another angle for looking at this, Mike, is the crest factor. That's simply the ratio of peak to RMS, but it's the closest thing we have to an objective measure of what's referred to as "punch" or "sparkle".
In your example values, your kick has a crest factor of 4.5 (18 / 4). That kick is probably going to sound punchier than one with a crest factor of, say, 2 (which is what you might get from compressing it heavily).
I include the caveat "probably" for two reasons. First, we don't know if the -4db maximum peak is typical or an anomaly, and second, we don't know the frequency content of the peaks. You could, for example, have a "ticky" beater sound typical of metal music that's not very punchy but has a high crest factor.
Vocals, OTOH, sound best with a low crest factor in most genres. A high crest factor might indicate a vocal with too-prominent "T"s and "K"s. Your lead vox crest factor of 2.5 is fine, but might suggest that more aggressive compression at least warrants a test.
You can lower the crest factor by either shaving peaks or by raising the average level while keeping peaks the same (depending on whether you want to adjust overall perceived volume or not). You can raise the crest factor by decreasing compression ratios. Tape sims lower crest factor. A transient shaper can be used to go either way (I like the one in Superior Drummer a lot for kick, toms and snare).
In my own projects, I look at the crest factor of the finished (mastered) product as an indicator of possible over- or under-limiting. I don't usually measure it, but can tell by looking at the waveform whether it needs a little more or a little less. My favorite productions tend to have a high crest factor, although what's acceptable depends on genre and style - a quiet ballad will have a lower crest factor than an aggressive jazz-funk piece.
Here's a slappy bass guitar with light compression and a modestly high crest factor. Peak is -8db, average RMS is -21db, for a crest factor of 2.6. Note how you can "see" the crest factor by comparing the thick area where most of the samples live to the spikes that represent each note attack. Depending on the source, the peaks may take on the appearance of grass or hair, and I've actually heard the term "hair" used in this context.
Here's the same clip with heavier compression to trim the peaks. The highest peak is now -11db, but average RMS has only dropped to -22db. The perceived volume didn't change much, but the crest factor as dropped to 2 and the track sounds less punchy.
