Ok thanks. Believe it or not I don't have any really good body parts (preserved remains of torso, and infested goblin arms, with default legs), but Ive managed a perfect goblin head and perfect dragon elf head. There was reloading on those ones but only 5-10 times each.
I think there's an additional kind of head that grants no skills, but extra hit points. I'm not sure it's worth the trade-off, though, even with the low damage all summons do. They're not really good at damage, just taking it. The Ranger's fireballs can do up to 150 sometimes, but the Dragon Elf can probably do more damage per second.
Goblin heads L10 fireball says 527 damage in 5 metre radius which is more like L9 fireball with less radius. Same with dragon elf head, the L10 spells are closer to L6/7 equivalents. I suppose getting arms that increase the damage helps.