Please Help - Battery Life woes of a Nexus Device! Am I missing something?

Hey People! Help Needed!

I recently bought a first Gen Nexus 7 (32GB) (it was a great bargain in my country at INR 10.2K ~ USD 150) for primarily non gaming use. I come from a 3rd world country so it was either this or NO tablet for me. The 2nd Gen Nexus tablet (32 GB) was costing approx INR 24K ~ USD 350

The first one came Dead on Arrival but booted after a couple of hours of charging. I was getting about 3.5 hours of (non gaming) screen time so I returned it. I got a new tablet as replacement which could switch ON immediately.

The replacement gives 5.5 hours of (non gaming) screentime over 2 days - which IMHO is less than expected.

Some data points:

a. Both the units were manufactured in March 2013 ( I am assuming Asus stopped production of First Gen Nexus 7 units after that)

b. I rooted installed Greenify to do away with background processes/services that consume battery.

c. Everything except Wi-Fi remains turned Off so there is no excessive battery drain.

d. I run the Stock Rom (4.3) with Franco Kernel (supposed to have better battery life & performance than stock)

e. Only 3 charge/discharge cycles have taken place so far.

f. There is not much battery drain in idle (max 5% overnight)

g. Performance is decent for a Nexus 7 first Gen (Antutu 14K+)

h. I use it at the minimum brightness possible.

My questions/ dilemma:

1. Is this kind of screen time (5.5 Hours) normal for a Nexus 7 first Gen considering the old manufacturing date

2. Should I keep it or Return it again to Flipkart (the seller) by reverting everything to stock (just hesitant about what I may get in the next replacement - as the previous piece had shitty battery life.)

3. Should I just sleep over it and enjoy with the limited battery life. Perhaps I can claim the warranty from ASUS in few months from now for a battery replacement. I know for a fact that it is going to be far from easy in my country.

Please advice! Any help would be appreciated! Am I missing anything that is supposed to fix things for me.