First of all... On your new K-5!
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Okay now, on with the questions:
1. Slow shutter speed noise reduction - do you now turn NR off or still keep it set?
I think this comes down a JPG /RAW, preference.
To which I'd add, it you are shooting in RAW, then you'd have nothing to worry about as you can get much better results in NR than what with in-camera performance.
2. High Iso noise reduction- at what iso (if any) do you apply cutom NR - I'm not keen on the auto setting
This is pretty much the same as the first.
However, if you are shoot in JPG then I'd highly recommend taking over of the NR controls yourself so as to avoid as many of the potentials downsides to leaving the camera trying to determine what's best(turns out the camera isn't that smart
)
3. Lens correction- am I best still using the PTlens photoshop plug-in or using this new K5 function
Given that you can do a bang-up job in post, I'd say leave it off. However, its the performance hit that likely makes this the most significant. Lens correction can really bog down the camera's performance.
4. RAW/FX button - what do you have yours set for
Have yet to use it
I know most people have already posted pretty much the same answers, but you have to ask yourself... do any of them offer a congratulations banner?