Dave,

I'd think carefully about your shooting style and the versatility of a zoom. Personally, I have the 85L, the 100L Macro, and the 135L - three excellent prime lenses within the range of the 70-200mm f/2.8L IS II which I also have. I can tell you that the 70-200 gets used significantly more than the other three combined.

Consider your shooting with the 55-250mm, or try the experiment...set it to 135mm and leave it there. Is that something you can see yourself doing all the time? You mention the TCs - say are shooting something with the 55-250mm at 135mm, and you'd be much better off framing it at 270mm. Would you have time to remove the lens, mount the TC, and remount the combo, and still get the shot?

I'm not anti-prime, far from it - I love my L prime lenses. But I had a 200/2.8, which I sold in favor of the 70-200mm. I had a 300/4 IS, which I sold in favor of the 100-400mm (partly for the extra 100mm, but also for the versatility of a zoom).

I did try the 2x on the 135L this morning, and it does make a noticeable difference in slower AF speed; of course, it's all relative - even slowed down like that, it might still be faster than the non-USM lens in the 55-250mm.

Generally with a prime lens, you're getting a faster aperture in exchange for less versatility. Compared to the current crop of excellent new zooms (70-200 II, 70-300 L, 70-200/4 IS), the IQ at wider apertures isn't probably different enough to matter in real-world shooting - and any would be a big difference from the 55-250.

I know what you mean about paralysis from analysis and wanting to get the best value. To me, at least, versatility is a huge value. That's why the 70-200 II gets used more. I think if the only telephoto lens I had was a 135L, even with the 1.4x and 2x TCs, I'd prefer the 70-300L for the same total cost. You rented the 70-300 L - was it hard to go back to shooting the 55-250 after that? If so, you'd have the same issue with the 135L.

If you'll be mostly in situations where you have no need to rapidly change focal length, the prime is great. For portraits, the 135L is awesome. For sports, if you're on the sidelines or otherwise can walk around to follow the action, it makes sense - but if you are in a seat, and the action is moving up and down a field, a prime would be a challenge.

As I said - you have a lens with a 135mm focal length. Set it to that and pretend it's a prime. Shoot what you normally shoot (even if you have to crank the ISO up to H to get the shots (to simulate the extra 2.67 stops you'd have at 135mm and see if the shutter speed is adequate). See what you think.