It is nearly impossible to keep a riser from flexing at least a little no matter what you do. It may not be noticeable, but it happens.
I don't build a bow without reinforcement.. necessary or not.. I certainly wouldn't build a 2 piece without it. The I beam is the strongest way. Will you get some spring back? probably not because you cut it after the glue up.
You can also reclaim some of the shape by adjusting the position of the two pieces of the riser before wrapping the sleeve. Not having built an all wood bow, I can't say that it would be necessary. The riser enforcement would be unless the bow is of very light draw weight.
Just my $.02
BigJim