You can try it up. No harm trying.
Actually, as long as your CDA number pair with SEUS server, then it should be able to update thru SEUS

Can change the player interface only not to say mod completely into walkman series.

There is actually a java program which you can install in your phone and the red focusing light will be on as torch

There is limitation for K800i macro.
You can't focus if your camera is too close to object.

Try not to be fanboy here.
I'd say new symbian works like a charm
UIQ of SE on W960i is part of Symbian

Ask it here.
http://forum.lowyat.net/topic/473043

AFAIK, W910i is running platform phone instead of symbian.

Doubt battery will drain fast after mod
It's still running the same thing just the interface and etc change which I kinda doubt will give effect to the battery.

Try check the back of your lens is it cover with dust. If not, try proceed to update the firmware. It might be firmware issue then

Not advisable to do so. If anything goes wrong, you might void the warranty. Why not just pay a little bit more to get a more stylish K810i ?

I feel more or less the same. Some firmware of K800i did make the photos look a bit lower then K810i.
Overall, both of this can said is the same

Corrupted EROM. You need to send to shop for a repair.
Is hardware problem, not firmware problem