Hi Phill,
I've had both the Pentax 50-135 and a Sigma 70-200mm F2.8 EX DG OS HSM. The Pentax is a lovely sharp lens but can be pretty slow to auto focus. The Sigma was fast and much more suited to auto sports, especially when turning off the Pentax SR and setting the in lens stabilisation to panning mode. I found my copy of the Sigma pretty sharp and not much behind the 50-135.

Brings back memories - my first bike was a BSA Spitfire

All now sold or reserved

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I'd agree with this. I've never had any problems with these cards.
I've used 32 & 16 cards in the K-3 (RAW/JPEG) - still plenty of space with those.
I think it's better to buy a card that's faster than the camera needs rather than have a slower card that could hamper the camera's performance, but any reasonably fast card is going to be just fine in most cases.
I've used My Memory a few times and their service and prices have been pretty good.
Remember it's the write speed that's the most important thing to look for rather than the usually quoted read speed.

D-BG5 Grip now sold