That lowering your graphics increase productivity in-game?

Define productivity.

Playing with everything on ultra won't make your gaming better.

The most important thing with your graphic settings is that you have stable FPS and no FPS lags in bigger fights.

I'm playing with models and textures on high and the rest on low.

If you have everything on low, you might have some trouble seeing cloaked units and forcefields, but buildings look more abstract.

But in general many players play on different settings and you can play good on any setting. You just need to find a setting you feel comfortable with.

Well I had the idea that if you lower your graphics, you won't be distracted much than having graphics on ultra.

It all depends on what you're used to. I play on medium/high, but if I lowered everything to low settings and played for about 15 minutes, I'd get used to it and it probably wouldn't bother me.

Distraction should have nothing to do with it. Or rather, if you're easily distracted because of the cool looking models, then that's something else to be worried about. The point of changing the settings is to prevent lag.

When you set graphics to the lowest setting, doesn't creep spread just "pop" onto the grid like in SC1? In this respect, it would be a bit easier to see exactly where the creep is and when, rather than judging by the creep spread animation. Other than that, I don't see any benefit, other than reducing lag if your computer cannot handle higher graphics. I consider it mandatory to at least have shaders on medium, just because the cartoon-like look of the lowest settings makes me feel like I'm playing a different game.

As far as the setup goes, I like to play with health bars always showing and having the command card buttons show their assigned hotkey. I think those benefit productivity, especially if you're not familiar with hotkeys or practicing with micro.

Constant health bars is a necessity as it allows you to easier see when invisible units are attacking you.

Nope, creep spread is always at the same rate. Lower settings just removes specularity and bump maps from the creep textures, also makes it not reflect light/shadows on really low settings. Creep spreads in a uniform manner now (as of patch 1.2? maybe?). Previously it would randomly 'grow' from the origin point. Lower/high graphics settings for detecting creep makes no difference - you have the 'Building Grid' and the default red/green tiles to show where you can or cannot build stuff anywayz.

I played SC2 without sound once... man I lost like 5 workers before I realised he had warp-prism DT'd me. Always wondered about how people got the hotkeys to show on the command cards... I assume there's a tick box for it... But late now for me haha - was a pain during the first few weeks trying to learn the hotkeys.

In short: Frames per second & gameplay stability are what really matters. Anything else is just a mixture personal preference and what your computer can cope with.