Hi cgoudie ,
I've thought long and hard about this over the years. Probably the ideal pair would be the FA*600/4 and FA*250-600, but you'd be talking mega bucks, and more importantly, mega weight (@ 13 lbs
each). . . and then you'd have to add the expense and extra weight of a very sturdy support system (also not cheap).
When making my choices, I considered practicality for me (a smallish 60 yo with some significant medical problems). My choices ended up being an FA*300/4.5 with a Tamron F 1.4x AF TC or the Pentax F 1.7x Auto Focusing Adapter for handheld shooting at 420/6.3 or 510/7.7 respectively at a bit over 2 lbs, and an FA* 300/2.8 with a Sigma EX 1.4x APO AF TC, P F 1.7x AFA, or both stacked, giving me 420/4, 510/4.8, or 714/6.7 respectively at about 7 lbs. I also have a Sigma EX 100-300 APO that needs some repair and both the Tamron SP 300/2.8 (60B) Adaptall 2 and a Sigma EX 300/2.8 APO which also work pretty well with the TC combos. With the K-7's increased AF sensitivity, I can actually push each lens a bit further with an additional 1.4x TC and retain AF capability, but it's really stretching things, both in AF performance and optically.
For me, a 300/2.8 (6-7 lbs) is the most lens I'm able to carry to most of my usual birding spots, and is well within the limitations of my current support gear (Amvona ATA 104T CF or Uni Loc System 1700, Manfrotto 468MG with a Wimberley conversion clamp, Wimberley Sidekick).
IMO, the P F 1.7x AFA is the key element to my longer-than-300mm lens strategy (I currently have 3 of these). In addition to the added reach, it gives me easy focus limiting, very fast AF, and very good optical quality. There are some compromises, but mostly in the quality of bokeh and possible multiplication of existing CA/PF, but as a dedicated amateur, these are both easily acceptable for the weight and very substantial cost savings provided. I regularly carry two bodies and both lenses with the necessary TCs plus a tripod/gimbal. I could never even consider this with any combination of the longer primes mentioned.
Here are a few examples:
K100DS, FA*300/4.5+P F 1.7x AFA (510mm) lens wide open
K-7, FA*300/2.8 + P F 1.7x AFA (510mm) lens at @f5.9
K-7, FA*300/4.5 + P F 1.7x AFA (510mm) lens at @f6.4
K20, FA*300/2.8 + P F 1.7x AFA + Sigma 1.4x APO TC (714mm) lens at @f4.8
And just for grunts, K-7, FA*300/2.8 +P F 1.7x AFA, Tamron F 1.4x AF TC+ Sigma 1.4x APO TC (1000mm) lens at @f4.8 focused using AF only with a max Av of f9.3!
K-7, FA*300/4.5 + P F 1.7x AFA + Sigma 1.4x APO TC (714mm) lens at @f5.3 focused with AF only with a max Av of f10.7!!
These last two were heavily PP'd. The others have minimal cropping and sharpening to offset the softening from full resolution downsized to 800 on the long side
Scott