Double update to compensate for the low posting rate.
These Large, 9 HD-ed aberrations, get 7th-level sorcerer casting as their main schtick, with Eschew Materials for free to make up for their general digitlessness.
In addition to that, they have above-average ability scores, always-on Detect Thoughts, immunity to mind-reading abilities, a small bonus on saving throws against effects that'd charm them, and immunity to poison.
They also have a natural bite attack that's more for mouthpick weapons than anything else, as well as a sting that delivers a save-or-lose poison. Too bad poison starts to lose its shine around these levels.
Can dark nagas keep up with 9th-level sorcerers? Probably: the lost spell level hurts, but they do get a number of goodies in return. That said, I don't think further level adjustment is necessary. Remember: an entire spell level is a large price to pay, and the sorcerer can by then have entered some fancy prestige class.
At first glance, the guardian naga is what you get when you take the dark naga and advance it by two HD. Sure, their ability scores are a bit higher, but they've lost most of a dark naga's special abilities and their poison got downgraded to constitution damage rather than instant loss. They can spit it a short distance, but I honestly think there's better uses of your action. Sure, you could spend feats on Deadly Spittle and Improved Spit to create cones of constitution damage, but why not just cast a spell?
There's one small thing in the guardian naga's favor, though: it can pick its sorcerer spells off the cleric list (plus the Good and Law domains') as well. That opens up a lot of options: even if a naga only selects sorcerer spells they still get to use all cleric-specific items (such as those tasty wands of lesser vigor).
Still, the lost PrC levels are enough to balance this compared to regular sorcerers. +0 LA.