Well with some experience while driving it (I do not own an i20):
1. Does the car is felt underpowered?
Not at all, people with lighter vehicles do comment about it but then they have already made up their mind because i20's power to weight ratio is much lesser compared to other hatchbacks (read Swift)
2. Does the lag shown in 2nd/3rd gear irritate?
There is no lag, that I felt. Maybe asingh can clear that up.
3. How does stearing behave at high speed - does it remain too light?
I drove at around 80kmph, it was pretty stable, not too light.
4. How do you feel the suspension at bumpy roads ?
It is a luxury car, I couldn't feel any bumps
5. Is overtaking comfortable at highways or you need to prepare in advance before overtaking?
No idea about it. I mean with fully loaded (weight)
6. How much it struggles on steep inclines?
No idea.
7. At what speed do you drive at highways and what is the mileage obtained at this speed ?
No idea
8. Is the car suitable for long drives?
It should be. I drove my Figo all the way to Hyderabad (from Bangalore)