Haven't tried the teleconverters myself, but I will mention that if bag space isn't an issue, the Gs-1 200/250 are usually very affordable. The 250 is especially cheap (like <$200). I think the 200 is a touch less common (I think it is also 1yr newer?) Having to use 82mm filters for the 200 is a bit ugh, but it is not much bigger than the 150 and if you want, at this focal length you could almost certainly use a step down to 72mm ring. I had heard the 200 was a better lens which is why I bought it and squinting at the mtf, it does seem a touch better, but I doubt you'd notice in practice. At 970g, the 200mm is a lot lighter than the 1200g 250 though and that you will notice in practice.
I wish I could buy the 500, just as a collector of things. My word, that lens on paper sounds incredible (but is v rare and $2k+...)
I have the 65mm and while I do use it, I kinda find it eh to use. I think the focal length bothers me a bit. It is not very special feeling. I think the 65-100/110-150 does make sense, buuut that 50mm does seem just a bit special.
It is a bit heavier and yeah it takes 95mm filters, but I think it is one of the real gems of the system. I shouldn't look so much at mtfs but that center performance is bonkers even compared to the other PG lenses. (
http://web.archive.org/web/20041206063145/http://www.tamron.com/bronica/prod/gs50.asp). I wish my copy didn't have a touch of fungus, broke my heart when I learned it couldn't be cleaned off even if it probably has no impact on performance.
But darn this whole thread has made me want to use my gs-1 more. Gah!
Stray pro-tip: Once you have a back loaded and on the camera, remove the dark slide and keep it in your bag until you need to change backs (WHY DID YOU NOT GIVE US A DARK SLIDE STORAGE SLOT BRONICA JEEEZ). The backs have 1 little flaw where they can open slightly easily if you leave the dark slide in.