I have the 2014 XPS developer edition and can get 10 hours with modest screen brightness. I get even more if I use a minimal WM like i3.

I find the two things that can really eat up the battery are screen brightness and having a poor Wi-Fi connection.

Try running Powertop and check out the tunables pane. You might even be able to reap some additional battery life.

There will probably be some hit, but probably not terrible.

I have the Ivy Bridge version; I probably get 4 hours running Ubuntu and perhaps just under 5 with Windows; this is not scientific though. I really should measure it.