Short answer, none. Short reason - the "working" part of the bow is just past the limb feather, where the laminates comes together to the limb nocks, so the riser should have no bearing. By this theory, it's all in the limbs.
Real answer, not too much and depends. A well made wood risered bow will flex / recoil from the pivot point (grip section) to the limb nocks, so by this theory the riser, too, is part of the working aspect of the bow. If your talking about a metal riser, probably not much useable flex there, wood risers depends on the type of wood and configuration.
I'll endorse Viper1's short answer with the following qualifications:
Takedowns have a more bulky and reinforced handle-riser which tends to make them heavier to carry. Some people think heavier bows are more stable to shoot.
Takedowns usually, but not always, have shorter limbs and higher risers than one-piece bows in any given length. While the riser might flex, it hardly does so as much as with a traditional English longbow so I'd be inclined to discount that contribution to total performance.
A takedown is handy for transport sometimes but probably not nearly handy enough to justify the additional cost in most cases unless the bow is five and half feet long or more.
One of the things I appreciate about my one-piece Attila, other than the light carrying weight, is that the riser is very low, so most of the 56 inch length is in the limbs. It would be very difficult to find a comparable bow in the same length that was a takedown and provided such a smooth draw and release.
I'm inclined to think anything invested in the handle is probably not going to contribute much to the launching of the arrow other than perhaps the aforementioned stability and that's a little problematic unless we're talking about target bows.
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