Edvinas:
1. Do you still have to use flash in churches with K-5? Presently K-5 seems to have one of the widest d.r. sensors/assemblies/picture engines there are out there...
2. I am not certain what kind of potential you're talking about? Not as a pun or offense - I really don't understand you. Are you trying to say that at this moment you have managed to extract absolutely everything your gear has to offer? If so, then perhaps a 645D would be a better step up, simply because the difference between cropped sensor and 645D is so much bigger.
3. That may be right. I am not entirely certain people would be as ecstatic about their FA limited lenses (I have all three of them) towards the (extreme) edges of the frame. It seems to me that these lenses were designed before modern DSLR cameras came out, so we may be or may be not up to some unpleasant or pleasant surprises.
Some time ago I had a hands on opportunity to compare Pentax K-7 and Sony A850. I wrote a
blog entry about that. Prior to that I was very much pro FF cameras as I thought they would offer some
significant advantages to the photographer by means of bigger sensor, "proper" AOV of the lenses, etc. Then came my Minoltian friend from Washington DC and we had spent some quality time together shooting and talking and generally having great time. In a nutshell, A850 is a better camera than K-7, but not by order of magnitude like some (including myself in the past) would try to present it. Further, when K-5 came along I have started to realize that personally (YMMV, obviously) I am getting to the plateau level of my technical skills, so that of course certain improvements would help, but not in a way of a new killer feature or a deal breaker.
Given the link to that article and a link to my blog in my signature you're cordially invited to have a look. It will also allow you to build an opinion about me as a photog and hence the applicability of my opinion to your situation.