No Rebug will not be likely making a CFW above 4.21 due to the fact Sony blocked DEX conversion's after 4.21 and I think it is why they chose that version to begin with since you can easily go from CEX to DEX in a matter of seconds using Rebug toolbox with their CFW and it is the last firmware that allows for DEX conversion so they went this route to maintain all the "extras" this CFW has to offer.
Rebug REX CFW is a dual CFW and has so many features and I am yet to see why I would even consider any other CFW period. Like 100% compatibility without the use of a bluray for dead drives or simply to preserve them. Built in save unlocker not to mention all the things one can do with the toolbox. This is simply a CFW that is making attempts but they all seem to be cosmetic when I am looking for functionality.
PS3 CECH-25001 4.40 MiralaTijera CFW v2 w/ Core 3.0.1 1tb iNt & 2tb eXT mM 4.30.00
Xbox 360 Trinity Slim with Dual NOR/HDD 250gb Stock & 1tb Squirt 360 RGH