Good point, but being that the Race doesn't benifit from the Elvish Dex bonus(doesn't get it), but does get the -2 Con IMO more than covers the "sixth sense" for secret doors, and possibly even the free proficiencies. The only odd factor is that it gets a bonus to 2 additional skills. Bluff and Hide
Infernal Elf-(+2 Dex, -2 Con, +2 Int, -2 Cha; Cold, Fire, and Electricity Resistance 5; Darkness 1/day; +2 Bluff, +2 Hide, +1 Listen, +1 Search, +1 Spot; "sixth sense" for magic doors; Elvish Blood; Native Outsider; Favored Class: Rouge) +1 ECL(+2)
When it comes down to it, if I were DM, this race's ECL would be based mainly on the kind of player using it. If it were going to be used for actual character depth and actual Rp'ing I'd say it's a +1 ECL, but if it were used by a player with no intent except for picking a race that give the best bonuses to his class, then it'd be a +2 ECL.
But Gromm, you made a good point.