A main problem with getting a new flash is that we often have just spent $$$ on the new body and a lens (or 2).
A second problem - when considering what to put on the K-5 - is it may be likely that one of the nice points in picking the K-5 is how compact it is. Don't really like the idea of putting a mondo flash on that sweet body.
My advice - 1) pay the bucks now and get a real good flash up front. 2) Don't worry about the size. Bigger IS better when it comes to having enough flash power (and in this case lots of great features - like a second reflector and an Auto-mode thryster sensor).
I went through numerous notions about what to get and finally just plunked down the $$ for the 58AF2. (I also got a BG4 knowing I would be shooting vertically). I am so glad I did! My 1st foray and specifically why I got it was taking portraits of the kid's swim team. One evening spent learning how it works, a couple test shots at the pool, and I was ready. The flash, and the camera with the flash - performed wonderfully. I have used it several times since then for other stuff.
BTW - the camera now seems small w/o the BG on it all the time.
Alot of us do it, but is somewhat confusing how we are so ready to dump $1200 on a body, more $$ on a fast lens, but then try to short change ourselves when it comes to important accessories.
The Metz 58AF2 is the best of similiar flahes, and is well worth getting. You will use it - if not for the band shots (wish i had it when I was taking shots of my B-I-Ls band), then for other things for sure.