I can't speak for the lenses that I have not had, but for 35mm, I have (had) the following;
- DA*300 (sold) - good - moderate price
- K300/4 (sold) - OK - cheap
- A*300/2.8 - good but heavy for it's size to a point I find it hard to use handheld but I am still able to walk around with it - ~2.5K
- K400/5.6 - good but does not close focus. Light enough to pack around on your shoulder. - ~$400 to $500
- A*600/5.6 - terrific but tripod mount poorly balanced (fixable with a "long lens support" or a long bracket) - Light enough to pack around on your shoulder ~ 2.5K
- K135-600 - nice but more of a novalty. Tripod mount not balanced and not good until around 400mm and up.
- A*1200 - terrific but expensive, very heavy, needing a studio tripod and can be very difficult to use.
- The 1.4X-L, 2.0X-L and the Kenko converters - I find cropping may yeild better results than using a converter.
If there is one thing I can stress is to pay more attention to the percentage of the jump in size of a lens than the millimeter increase. If you are already at 250mm I can't see much gain in going for anything less than 600mm.
I quite like the A*600 expect for the horribly unbalanced mount, but there are workarounds for that and if you're OK with manual focus, it could be an option. I haven't tried the FA*600 but I expect it might even be better. As mentioned above, you can also add a teleconverter, but anything over 1.4 could show a drop in quality so I tend not to bother with them.