It absolutely saddens me to have to rate Pentax lower than awesome but other photographers have to be aware so they don't miss a shot. The quality and durability of Pentax continues to keep up with me and my bird photography in rain, wind, sleet, snow, hot, cold, whatever. My arsenal includes the K-3 and two WR lenses which have never performed badly until I linked them up to this TC.
The 1st (and absolute awesome) lens wont even focus all together (smc PENTAX-DA 18-135mm F3.5-5.6 ED AL [IF] DC WR). But since the original plan was to extend the performance of my 2nd lens and maintain the WR integrity (HD PENTAX-DA 55-300mm F4-5.8 ED WR) it crushed me to finally come to terms with the TC's inability to perform at max focus. I tested the 300+TC combo using a monopod, tripod, shooting landscape and wildlife. And I even thought about firmware updates and that was taken care of. Some people had a better outcome with the TC so perhaps you may want to try it and return it if it doesn't perform. I did exactly that. Still loyal to Pentax quality and durability. Not sure what happened with this TC.
Raised the evaluation from two stars to four because Pentax is a great brand and they apparently had the information published prior to my purchase and I missed it: Compatible (AF-enabled) lenses for HD PENTAX-DA AF REAR CONVERTER 1.4× AW
•HD PENTAX-DA 55-300mm F4-5.8 ED WR - 3*
3* Could be out of focus with phase matching AF
Went shooting today without the TC and I miss the reach but AF is nice to have for birds in flight. I hope a future update can fix the problem.