I have heat treated red oak with suprisingly positive results ... makes a real bow wood out of it. I'd imagine white oak will also respond very well.
It is likely to change the tiller though, because it's nearly impossible to heat treat it uniformally, but the tillering should be really minor.
You could potentially gain ALOT of weight with heat tempering oak. I wouldn't be suprised if you gained 10lbs ... probably more like 5-7lbs.
If you do decide to heat treat, make sure you do it with the bow clampsed to a form so it doesn't twist etc. Also be casreful not to let the back get toasted, it can cause tension failure if the back is heat treated at all.
Good luck.