The 303 material does work really well, their new NCF power distributors are amazing imo.
To me, it seems like the biggest difference is the use of silver plated wire, this makes a massive difference in overall sound quality. The combination of copper and stranding of the wire sounds exactly like the current cable except for the silver plating on the UPOCC strands and 10g vs 11g. It sounds like you'll still get a dark character of sound which I think is a result of the carbon used in the sheathing, but in the current cables it adds a particular character, a "bloom" to the upper midrange which keeps it from being as neutral as I'd like. In general you'll get a sound that tends towards warm and dark with the top end Furutech PC and SC. This works great for a lot of modern systems using big SS amps and hard-coned drivers but they're not the last word on neutrality or clarity, they are fairly colored but they do maintain enough resolution to achieve a satisfying result. I seriously doubt their new cables will be anywhere close to a pure UPOCC silver cable in terms of overall clarity, neutrality and resolution though.
The new plugs will also make a big difference I'm sure, AC power plugs make a huge difference in general and the previous FI-50 NCF are in a league of their own imo, it will be interesting to see how the new plugs compare but I'm not sure when they will be available. Furutech USA has had no info to share except it might be a while, and this makes sense as I'd bet Furutech will like to keep them exclusive to their new cable for a period of time before they are released as the successor to the current FI-50 NCF plug.