I think you're overestimating the amount of power you save in going to a smaller screen.
You got a 20Ah battery pack? You sure you don't mean Wh? I assume you mean something like this:
https://www.amazon.com/PS-Vita-Portable-Charger-Sony/dp/B006JKARP8?ie=UTF8&ref_=cm_cr_pr_product_top But most people don't want to use an external battery pack with a gaming handheld. Or they certainly don't want to have to.
And even then at 10W you'd last two hours on a 20Wh battery.. and I doubt a portable PS3 would only consume 10W total.
10W is also some pretty serious heat for such a small form factor, you'd probably need active cooling and ventilation.