Sounds to me like it's technique as the kit lens you have shouldn't produce noisy or blurry images. Blur (rather than softness) is down to too slow a shutter (causing motion blur) or the AF
not quite locking on, and grain is produced by the sensor, not the lens. The 50D is pretty good at controlling grain, even at ISO400, so I suspect it's something you're doing as opposed to a fault (although based on recent experience a lens fault shouldn't be ruled out). Are you using full-auto modes by any chance?
I used kit lenses for a couple of years before upgrading to L glass and had no issues with them. In fact, the first kit lens I had was optically inferior to your 28-135.
From what you are saying, I doubt buying a 70-200 f/4 L will solve your dilemma. The 28-135 has IS
, whereas the 70-200 does not, so blurry images should effectively become more commonplace still.
Not trying to sound condescending but I would consider learning a little more about aviation photography before jumping in with both feet and buying an expensive lens. L glass will help with image quality only if the photographer is competent enough - in other words, if you aren't getting good images with cheap gear the expensive stuff won't help.