Originally posted by Sandy Hancock Maybe it was just my copy, but the DA*200 I had for a month produced substantially more lateral chromatic aberration. Anyway, if you're after reach it might be disappointingly short - put the teleconverter on it and you have a 280/4.0 - still short of what the 300 offers on its own.
But the 300 ƒ4 never offers ƒ2.8 in any configuration. The 200 is the longest ƒ2.8 lens currently made by Pentax.
It may be that a K-70 or K-P can make the 55-300 more useable, but everyone upgrades some day, And 2.8 will always have advantages over ƒ4 even when used with a K-P.
The one question to be asked is, are you going to shoot kids sports indoors or under the lights. If you are, you want 2.8. You are going to be struggling even with an ƒ2.8 lens.
If not, all outdoors in good light, ƒ4 is fine. Personally, I seem to be saying this a lot lately, I prefer the TC on 2.8 glass.
Since you have the 55-300 and 1.4 TC you know what 300 is like (and probably 420). If you are happy with 300 ƒ4, 280 ƒ4 will do. If you want 420 you pretty much have to go with the 300 ƒ4.