UPDATE:
For me the STX was atrocious. It didn't mesh well with me or my eyes at all on the 65 module. Compared to the Meopta S2 everything felt dull and less engaging. Eye strain was imminent and persistent and I decided that I am done with one eyed viewing from now on!
After returning it I went out to get the balance rail today and on the way in I realized I might look into trading in the 65 towards the 85 since I didn't keep the STX, so I had some $ to spare and the BTX loses some light compared to the other ocular modules.
Went in, got a fair trade for my 65 and case towards the 85 and Balance Rail and headed home.
I was in for a nice surprise, I was pretty happy with the BTX and the 65 but imaging was a bit grey-ish/dull in the afternoon shade.
With the 85 I find that the whole image sparked up a notch. Spent a good hour outside (until fingers went numb from the cold) and the imaging from the BTX / 85 was very rewarding. Checked out the local deer herd, looked at some birds nibbling at the spring blossoms. The image is noticeably brighter and very crisp, the bird viewing was the best I have had so far with any spotting scope - with incredibly crisp detail and a remarkably calm image.
I think in my case, going from the 65 to the 85 was THE significant upgrade. With the 65 it felt nice, but, a little compromised with the BTX.
That is why I looked at the STX, to get more versatility out of the kit. Now I feel I don't mind the absence of a zoom and perhaps I will even try the extender once the refund for the STX balances up the expense account.
Also, together with the Pure NL 8x32 the imaging is more similar than different and the pairing is great. The ONE drawback is that the BTX just barely fits my backpack with the balancing rail. Still, having gone this far, looking at a slightly larger backpack is not a dealbreaker.