Since a hero point can be used to bolster an attack roll or save by 8 (if announced before rolling), would you, as a gm, allow it to increase the save dc of a spell you are about to cast by 8 as well? You aren't lowering the target's resistance roll...you are increasing your own dc by 8 for one spell that HAS to get through. Seems equivalent to an attack roll...
Definitely not +8.
I might allow a +2 or +4.
If you're adding +8 to the saving throw DC then I'd say no way. If you're adding +8 to your roll to overcome SR that would be ok. If allowing it will make the game more fun then go ahead, but you might create a bad precedent.