So there's no way to keep the requirement for Aasimar / Tiefling in tact when the resistance is applied to an item? If it imparts the 15/- aspect, why could it not impart the Usable by: ...... restriction? This comes from a complete ignorance of the builder, obviously.
Or you could rely on the player base to police themselves up (as oneself policing him/herself) I remember the white stag armor not being usable by ranger. Mechanically it is, but there is a warning on it that states that if a gm finds you with it, the ranger could lose his powers. As far as I know, I have never seen a ranger use it and/or lose their powers.Kind of the same with the arrows made from the special bows, like the trapping bow and the ilsare eyes. They are usable by certain type of character, creates a special item and is noted on the bow as it is used to create the items (and in the description) not to give the arrows away.Something similar could be done I guess, but with different repercussion. The Item could be activated by the aasimar or tiefling, but should not be given to any one unless of the same race?
Beyond these two factors, there's still the more balance-oriented question of whether or not we should have what amounts to race-specific stacking of something that NWN doesn't stack. Given how things are balanced here overall, that 5 extra points can be more significant than numbers alone suggest, which is likely why our resistances cap out at 10/-.
Aasimar and Tieflings already have some pretty significant advantages, and while they're ECL 3, there have been some pretty high-level characters of these subraces. Adding yet another doesn't necessarily seem appropriate.
As far as overall balancing issues.... I see very little issue with this, as the spells now have uncapped capacity to do damage. The only instance this may impact would be for traps, or something with a fixed amount of damage.
With the scarcity of Aasimar / Tieflings on the server...
just a random thought and since I have no idea how the subrace skins are made this might not work, but what about adding the feat resist energy to Aasimar/Tiefling? since according to Resist energy - NWNWiki, the Neverwinter Nights Wiki - your guide to the game of NWN it stacks with any none feat source of damage resistance