I own the ZD300 f2.8 which I use lots w/ the EC14 (amazing IQ) w/ an E-M1. Have been considering the SWD 50-200 & a EC20 but keep pulling back. And I've been doing lots of plane shooting both full size & RC models. Here's my thoughts on the topic & hope they are helpful.
You may already know all this & if so i apologize for taking your time. Note these tests are with the focus limiter on & the 50-200 does not have a limiter. It will therefore hunt more & be slower. The nice thing about the SWD you got is its mechanical focus ring. Very valuable since the OMD does not have C-AF+MF. So technique will likely be important for getting the best AF speed out of your lens - you can find several posts about this both here & over at DPR. Try using the focus ring to pre-focus on planes before you half-press the shutter button or start a sequence.
As for comparisons w/ the m4/3 75-300, I've seen reports that it is faster for C-AF than the 50-200 & others that it is slower. My conclusion is it probably depends on the subject. As for IQ comparisons w/ the EC14, there again, I've seen comparisons both ways so it depends on your particular lenses which is sharper. But overall that means the 2 lenses are close for both IQ & AF.
I love using the EC14 on the ZD300 & the pro wildlife photog I bought the lens from never took the EC14 off. I rarely do either. He tried the EC20 & found it was just too slow to be practical. I've used several 2x TCs on older but very sharp super teles & have always been disappointed. Heck, I've even stacked a 2x w/ a 1.7x TC - OMG. An issue rarely discussed is how very hard it is to nail the focus when using a 2x TC using either MF or AF. The ultra high precision required when using a 2x TC means even if the optical performance is acceptable, frequent small focus misses will happen & mean lots of unsharp images. This is the reason I've held off buying an EC20 - doubt that even w/ a great lens like the ZD300, I'll be disappointed.
But I've seen several images from 50-200 owners using the EC20 & for web images, they were at least usable. But I've also seen some using just the EC14 that were definitely soft. I suggest you try out your new 50-200 to see if you got a real good one before buying an EC20 since they aren't cheap even used.
What ever you do, you'll have lots of fun shooting the new setup but you may have to adapt your technique some.
i just got a 50-200 SWD as well, but do not use it with a TC. I only have the E-M5, so the lens does tend to hunt quite a bit initially. That said, the images are far sharper and have better contrast than the 75-300 mk ii which I also own, though that lens has much faster AF on the E-M5. What i do now is use the 75-300 when I know i need fast AF and the light is decent; the 50-200 SWD is great for images in low light and with fairly static subjects.
A few samples below which I have posted elsewhere on the site:
I have the 50-200 SWD and the 1.4 teleconverter. AF speed is perfectly acceptable with the E-M1, albeit not as fast as native m43 options. The setup is large but manageable, similar in size and heft to the 40-150/2.8 but it does extend and there's the thickness and weight of the adapter to consider.
Most recommendations I saw were to avoid the 2x TC for the 50-200 due to softness and so so performance. The 1.4 works great, particularly stopped down a touch. Good loo lens technique is still a must to get the most out of this and any long lens.
I have both the ZD 50-200mm SWD and the EC-20 teleconverter. I think it is a perfectly useable combination in good light. I don't really see any difference in AF speed with or without the converter when using my EM-1. It might be a tad soft compared to the EC-14 but when you are shooting telephoto that extra reach can make all the difference and there is a big difference between 40% extra reach and 100% extra reach albeit with the trade off of the additional one stop of light loss. As the OP said, he can always resell it and recoup most, if not all, of his investment so why not give it a try? KEH has one for $352 with case and caps that seems like a reasonably good deal.
Where are you finding new Olympus gear in Houston? I haven't been camera shopping lately, but I keep getting the bug to at least go look. HCE on Richmond is a long drive from Galveston County, I am have not bee there for a while. Enjoy Wings Over Houston! I'm headed for the Brazos River Valley!