Are you seeing soft focus/OOF with the 50mm or with the 70-200 or with the 15-85 or with all of them?
The 50/1.8 has very iffy autofocus. Folks trying to use Reikan FoCal to set Micro Focus Adjustments usually get frustrated adn just give up on that lens. It doesn't focus consistently (or quickly, or quietly, or very accurately). But, hey, for $150 or less, can't expect much! And when you nail a shot with it, it's pretty amazing for such a cheap lens.
You should have better luck with your two zooms since they are both USM lenses.
On all your lenses, you would be better off simply taking off the filters most of the time and only installing them when they are actually needed. It's silly to expect a thin piece of glass to provide any sort of real protection. In fact, just the opposite can happen... I've seen more than a few lenses scratched by broken filters!
Use a lens hood to protect the lens when shooting... and a lens cap to protect it when not shooting. Both do a better job than any filter can.
Putting a teleconverter on a lens will always degrade image quality to some degree. With better quality TCs and weaker ones such as a 1.4X, it's not a lot. Sort of depends upon the lens, though, too. TCs in general work better with prime (simpler) lenses than with zooms (due to their complexity). Your 70-200 should be okay with it though... it's a good lens to start with so some minor loss of IQ when a mild 1.4X is added isn't going to hurt all that much. Your other lenses simply aren't pracitcal or purposeful to use teleconverter on (even if it could be fitted, which the EF 1.4X II can't)
If you are seeing soft images out of the 7D with all your lenses, it might be your post processing. It uses a fairly strong anti-alias filter and images from 7D tend to need more sharpening than a lot of other models. If you shoot RAW, in particular, you might want to apply some more sharpening and see if that helps.
There's also some discussion about the high density 18MP cameras being more sensitive to camera shake than lower resolution models that came before them. Canon even published a white paper about it a while ago. Just as a precaution, I tend to use my 7D set at an ISO a little higher than other cameras, to insure a slightly higher shutter speed that will counteact any additional sensitivity to camera shake, which can certainly make images look soft. It really shouldn't be any different from your Kiss, though... since that's also an 18MP APS-C camera.
Finally, probably the best suggestion I can give you is to scale back the 7D's AF system to something really simple and straight forward at first.... Such as using only the center AF point. A lot of people have problems jumping in and trying to use some of the "gee whiz" focusing modes inapproptiately... and see focus problems as a result. 7D is very fast focusing, so it can go out of focus just as quickly too when you set it up wrong and don't keep the focus point right where you want it. It takes some time to learn to use it well. Experiment with different focus modes a little at a time and use the multi-point modes very sparingly. I use Single Point about 85-90% of the time or better. Far less frequently, but my second most used is Spot Focus, which is a high precision mode that's a little slower... and also only uses a single point. I use Expansion and Zone pretty rarely and absolutely never use All Points/Auto Selected. Those leave too much up to the camera to try to figure out where I want things focused, and IMO are just asking for missed focus issues.