Ok Roy, this is my take on fixed vs zooms. If you've read some of my posts will will know that I am just getting back into birding after a long lay off. I'm still working and I work six day a week, so I don't get that much time to do anything. I'm a keen cyclist and I ride my bike on my day off, Sunday. That's the reason I didn't buy the Kowa, it simply wouldn't get used enough at this moment in time.
Because I have had the Optoltyh for more than 20 years and the zoom was very poor and I mean POOR. I decided to take a look at the latest scopes on the market and I went to Martin Mere a couple of months back and took the Opto with me. I did a comparison with a Opticron ES80.ED with the SDLv2 ep and Swaro ATS80 HD with 25x50 zoom. The zoom on the Opto was a non starter, so I compared my 30x on the Opto with the zooms. The 30x on the Opto I thought was equally as good as the Opticron when set at the same mag, but not quite as good as the Swaro. Remember, I'm comparing fixed 30x with zooms. I then decided I wanted a zoom, it had just been demonstrated to me that they were as good as my 30x and possibly better, so I bought a Baader zoom to fit to the opto and this was a huge improvement over the Opto zoom. However, I hastily sold my fixed Opto ep's which left me with the Baaders zoom. By this time I had got the bug, so I decide to explore more recent options. Ideally I wanted a smaller scope and I tried a Pentax 65mm ED, which was very good, not sharp enough at max zoom with the Baader, so I was back to looking at bigger scope. I tried the Kowa and the Opticron side by side and there is no doubt the Kowa was the better scope when using the zooms. I also tried a fixed ep on Opticron 32x on the ES80ED and to be honest I was quite taken by it's sharpness, equally as sharp as the Kowa set at 30x. So, concluded that my old Opto 30x ep was has good as the Opticron zoom set a 30x. However if we had put a fixed 32 in the Opticron side by side with the Opto, when I was at Martin Mere then I would have known the the Opticron was better.
My view is, and it is only from my very limited experience, that fixed ep's are better than zooms, even top zooms.
Would the Opticron with a fixed 30x ep be better than the your Kowa TSN4 with a fixed ep?. If the Kowa had the latest 30x ep I think the only difference would be the newer scope would be a tad brighter, I don't think there would be that much difference in sharpness and contrast
Having said all this, there is more to a scope than sharpness and contrast. The view though the Kowa 883 is really nice and once you've been there, you do miss it.
Hope this helps.