Can the length of disc brake hose influence braking power? I recently installed new brakes (Shimano SLX 675), shortened hoses for front/rear brake, done bleeding but even after that rear brake lever feels much softer than front. (I had identical issue with old brakes (M446) which acted in the same way and that is partially the reason for asking this question). 

New brakes work fine, they have nice stopping power, there are no signs of oil leaking etc... It is just that rear brake lever feels much softer (and I don't think it is due to lever reach adjustment; it isn't nearly reaching handlebar). 

Can the longer length of disc brake hose (rear breake) influence the 'feel' while braking (assuming brakes are correctly installed and working properly) or there is something else causing this 'problem'?
This sounds like an incomplete bleed. If you hang your bike vertically, try pulling the rear brake lever after it has sat overnight (front wheel up). if it feels extra bad that confirms it. Or if you notice that the brake feels better after a few 'pumps'. 

That said, I had noticed with my SLX brakes that the front and rear had a different 'feel' possibly. Not that they acted differently at all, but felt different through the stroke of the lever. I thought it was possible it was the factory bleed, which wouldn't be the case for you, but perhaps there is something to it. Or possibly a difference in friction in the Pistons in the caliper or lever somehow. 

On my Jamis dragon actually (separate M675 from above), I think the front brake ended up with too much oil from the factory bleed on the front. I couldn't get the pads open enough to not rub the disc. I put a bleed block in the caliper and opened the bleed screw on the lever and that pressure alone pushed a bunch of oil out. I closed it up and everything worked as it should have. Possibly something similar has happened in your case, but too little oil? (this could happen if the Pistons were too far retracted in caliper/lever not fully open during bleed)

side notes: Of course the front lever is going to provide more stopping power as it has greater mechanical advantage. Also some bikes have a larger front rotor. And never rule out quality control.