Reaktor sync was succesfully fixed in Renoise 1.28 build.
So now to play sequensed Reaktor ensembles - need to put note (of Reaktor VSTi) in channel and then press play in Renoise - Reaktor must start with Renoise simul.
And it gonna redirect Reaktor’s sequenser from exact position when you playing Renoise’z page or song from different start positions.
I hope it will work with Reaktor Session, cause i tested it only with Reaktor 4 or 3.