Didn't see one in "shoot outs" so I was wondering in good light what is the difference in image quality between the 2. Thinking about the 2.8 but don't really using that length a lot, that why I have the f4/5.6 version.
In good light you may not see a ton of difference. However in less than good light, the f2.8 is a whole stop faster, so you can still possibly get the same exposure without having to bump your ISO. Also in general the more "pro" level lenses tend (usually but not always) to have other benefits such as better build quality, better optics (coatings, types of glass, etc.) and of course weatherproofing.
If you are always going to be in good light and good weather, then what you have it likely fine. But if you want to shoot in more challenging situations then upgrading to the f2.8 model may have definite advantages.
If you shoot landscape - not worth bothering. If you shoot portraits - probably you spend the money better on a prime. If you shoot events on a regular basis - 2.8 zooms at least are a good compromise.
Since you don't shoot that zoom range a lot and you are shooting in good light, I would save the $$$ and get the slower lens.
If you shot it a lot (enough to justify the pro lens), or had to shoot in low light, then I would get the f/2.8 lens.
So far, I have found that m4/3 lenses have pretty decent IQ. At least "good enough" for me.
I have and use the pro lenses, when they provide a function that the consumer lenses do not; f/2.8 lens for concerts and 12-100 for zoom range.
But I use the consumer Olympus 75-300 rather than the pro Panasonic-Leica 100-400, cuz I don't use that range enough to justify the $$$$ on the P-Leica lens.