Assuming you can use the PAO trick to become an Illithid at lower levels (so you can get into Illithid Savant at 8), the Ritual of Unlearning from SS is crazy. It allows you to become a creature of X ECL once you reach that ECL on your own.
What you are supposed to do is start the ritual at level 1 and level up normally, paying as you go until you reach the ECL of the creature you want to become. Then you trade in all the class levels you've earned since starting the ritual (you keep whatever levels you had when you started) and become a 1st-level whatever. What you actually do is level normally until you reach the ECL of the creature you want to turn into (15 in this case) and then start the ritual. Since you keep whatever class levels you had when you started the ritual, you are now a Mind flayer with 15 class levels that is ECL 15. So, go X 7/Illithid Savant 8, using PAO to get into Illithid Savant initially. Once you hit ECL 15, you do the Ritual of Unlearning, become a Mind flayer, and then continue leveling normally. In the end, you'll be a Mind flayer X 7/Illithid Savant 10/Y 3 that is ECL 20 (and yes, you have 28 HD. Don't ask me how they thought this crap was supposed to be balanced).
Now, the main downside to this is that it is expensive. To turn into a Mind flayer would cost 256,000gp. Still, this is hands-down the best way to have your cake and eat it too.