Apologies but that all sounds like rubbish to me - like a man who is obsessed with 'kit' rather than taking photos.
I don't know the OP so have to reserve judgement. He might be a professional and will shoot me down for my criticism.
Do you enjoy taking photographs - do people enjoy looking at your work?
Are people who see your photos 'getting really tired of seeing the difference in image quality' too?
In all fairness I can understand your interpretation of my points, but I would respond by saying:
Spending over £800 on a lens is not a decision I take lightly (nor does my wife !). As a result I wanted to get a collection of opinions to help me work through and reach a tentative decision.
My first SLR was an old Ricoh KR10 and I spent many happy teenage years printing BW and cibachrome prints in my parents downstairs bathroom.
The reasons I now have an SLR is because I like the photographic flexibility of what a modern DSLR can offer. I can experiment in 100s of ways with no costs in film, chemicals of paper.
The primary reason I take photos is passion. Pure and simple. I love the ability to play with time and perception, to capture something that the eye can the look at in its own time.
I am not a pro by any means although I do occasionally get paid to do some portraits. Having a lens that handles well makes taking a photo more enjoyable just as riding a good mountain bike feels better than a cheap one, but also importantly a good lens will allow (my own skill not withstanding) better images to be captured. More details and sharpness allow larger prints, better colours allow images to just 'look' better.
Having said that I do understand that an image is not all about sharpness, bokeh and colour contrast.
The image I look at most is soft with blown highlights taken late at night hand held with a single table light as the only light source, the power is the image, not the sharpness or the bokeh or anything like that.
It is a very simple image, in it, a close up of my mother who has just finished a course or radiotherapy, it's 2am and she is sat there staring into the camera with a exhausted sadness, her eyes are haunting. Within 8 weeks she was dead.
I'm sure this kind of analysis is useful, but use it in context 
If you can think of occasions when you didn't quite have a short enough focal length (which I regularly experience personally) etc, then this must be considered as well 
, now the 400 feels like a cheap toy.