dont forget falling behind for level based feats and the stat point
I think that +6 to all stats for a +2 level adjustment is probably just about right. Everyone takes a hit, everyone gets a bonus, and the bonus is actually worth enough that people might take it now and then for more than purely flavor reasons (people play pet rocks whose only ability is to shout derisively for 'flavor reasons').
Effectively it amounts to more natural talent but less training. Less hd, less base whatevers, less max rank for skills (this is actually a pretty big loss although it is easy to overlook), less attacks, less class abilities, etc.
Some classes will get a bigger boost than others, that is the way of everything. Overall though I think it would work out pretty well. Primary spellcasters lose out on a full level of spells but gain higher dcs and are tougher to kill, trading offense/defense/utility for a different form of offense/defense/utility. Dc's will be ahead, effects will be behind, that sort of thing.
Hp wise the crossing point will be earlier (for awhile one will be behind then the other will be behind) but with being 2hd behind any con boosting items will be slightly more effective for the guy without the template, pushing the transition point further back.
Still, I dont think it is a 'must have' or 'must not do' for pretty much anyone. It is a tough choice. Some builds make the choice easier (some still fall into the definately or definately not camps) but for the most part.. being behind by 2 levels of class abilities can definately hurt at times.