If you have the T5 I wouldn't upgrade...
can I ask why you need to upgrade and what are you looking to do with the Sx900.
The Sx900/Genos styles are easily obtainable and converted for your Tyros units T4,T5,
Also maybe a switch to add a different arranger to your setup to give a bit
of mix to your Yamaha wouldn't be a bad choice also, Ketron, Korg, Roland, etc,..?