That sounds like a decent option despite their ideologies being starkly opposed to my own (xenophile, fanatic materialist to a xenophobe slaver).
I'm still curious what would happen if I took those systems as a condition of peace. There are a couple juicy ones that he snatched right around my borders while I was expanding laterally.
You get the planets right away if you take them as a condition of peace. Though, if you're not taking everything, his empire would still be around and out to murder you. They'll probably shop around for allies.