Planetouched aren't necessarily humans (but peopel tend to assume they are for the sake of simplicity). And planetouched aren't half one thing and half mortal. A half-tanar'ri isn't a tiefling, after all, it's a cambion or alu-fiend. Tieflings have an outsider ancestor that's a bit further back in the family tree. Maybe none of the generations in between even showed signs of the planetouched blood. That's how quirky these things are.
I've been thinking of doing something like this, but there's a problem with it: it disregards the abilities of a LA+0 race. Humans and elves, for example, have plenty of abilities, but they still get their full ECL's worth of class levels. You need a system that gives a template which, when applied to a human, gives you an approximation of the tiefling race.
By your system, I should be able to get a "human" tiefling by giving my character all the standard racial abilities of humans and tieflings, which would, of course, make him woefully overpowered.
I'm tinkering with a set of specific templates to handle this.
I would think - for the sake of even handedness in using this rule, that the human hybrids would also get their bennies. Numerically that would make the characters just a smidgen better than using the race as written - but it sounds like playtesting is supporting it. How long has the game gone on?