There are several copies of the 150/2 on eBay starting at $1270. There are also two copies of the 300/2.8 for sale starting at $2100. These lenses are available, if you want them.
It would probably use the same 77mm filter as the 300mm f4. This would be a lens that would fill the zoom gap up to the 300mm f4 for me as well as give an f5 - 98-350 with the MC14 or an f7.1 - 140-500 with the MC20. Unfortunately it would probably be very expensive.
If a 70-250 f3.5 lens actually gets made, and if it takes at least the 1.4x TC, if it has Sync IS, and if it isn't too frighteningly large, heavy or expensive (yeah, right), then I'd be very tempted to flog off some other stuff and buy one.
However, I expect one or all of the above factors will scare me off, in which case a 40-150 f2.8 v2 with Sync IS might be very attractive.