I've seen it discussed before and it seems to depend on your market. If you're in a spammy SERP, more experienced searchers will skip the first page altogether. With less experienced people, they'll look each result going down the list.
I suppose the major factor are how experienced are your users and how much spam is in your SERP.
I'd say stay at 10 and keep trying to work your way up above the fold on page 1 rather than work your way backwards. But that's just me :)
You should look at your logs and see how much traffic you get for "kw2 kw1" while you're in 11-14 and proportionately compare to "kw1 kw2" in 5-6 You can usually get a good estimate of which one will pay off most if you use reasonable proportions. Find that out and then go at whichever one more likely to score the best traffic and don't worry about the variant. Or target the variant with another page. You've got lots of options, play around a bit :)
I wonder if Google and other SE's don't actually rank you in a linear fashion.
That is, in their place I might put someone at the top of the second page if they were a more targetted result than results #8-10 on the first page. This would reward sites based more closely on their expected utility for surfers, and get more ads too.