So...if Empy was collective psyker experience of hundreds or thousands of shamans and druids...and Ynnead is gonna be collective tota experence of however many Eldar have been placed into Infinity Circuits...
Not as powerful as you might expect. Remember, the shamans used the power of reincarnation to live over a hundred thousand years before they became the Emperor in a time when the secrets of the Warp had never been easier to access since before the War in Heaven. Current Farseers - with the possible exception of Eldrad - are all less than ten thousand years old. The human shamans of old actually had much more Warp knowledge and experience (and thus power) than the Eldar of today, as surprising as it may be to some.
What they're trying to do now is what the Imperium did to create the
Siegfried - using a certain piece of technology and cobbling it together with another to do something it was never meant to do. The purpose of the infinity circuits wasn't to create a god, and while they're capable of doing that, the only reason the Eldar are using the circuits to do so is because that's the best thing they have in order to accomplish their goal, lacking the knowledge of the shamans to do it right.
In conclusion, not only do they not have close to the same level of psychic ability and knowledge as the shamans, but they're using a sub-optimal method to make their god. The end result will likely have enough power to beat Slaanesh, as intended, but only by brute-forcing enough Eldar souls to increase its power level, and even then it's an inferior product compared to the Emperor since it's a god with no material body.