General dynamics did experiment with sea vulcan 25. GAU 12 25mm (5 barrel) fitted to a similar chassis to vulcan minus the radar and was remotely operated rather than automatically directed. Intended for surface action rather than true ciws. But 540 rnds rather than 1000 ish of 20mm. No takers at the time.I suspect that there would be a bit more to it than that. Consider that a late model Mk 15 Phalanx (Block 1B) has a displacement of ~6.2 tonnes, when kitted out with the M61 Vulcan with a 2 m barrel.
Then consider the impact of replacing the 20 mm M61 Vulcan rotary cannon with the 30 mm GAU-8 Avenger rotary cannon (normally found in the A-10 Warthog...)
Given that the basic rotary cannon weapon is both larger and heavier when switching from 20 mm to 30 mm, the entire Mk 15 Phalanx mounting would likely need to be redesigned. Also due to the greater recoil forces involved in the upsize from 20 mm to 30 mm. the structure of the Mk 15 would likely require some sort of recoil dampener and/or reinforcement. All of this would likely drive the weight of the finished product beyond the current ~6 tonnes for a Mk 15.
Another thing to consider is the Goalkeeper 30 mm CIWS, which already uses the GAU-8 Avenger rotary cannon... A major difference between Goalkeeper and Phalanx aside from calibre is that Phalanx is non-deck penetrating while Goalkeeper is deck penetrating.
My interpretation of the above facts is that, short of a new rotary cannon being designed in 25 mm or 30 mm that has a similar size footprint and recoil forces as the M61 Vulcan, weapons system designers are not exactly spoiled with viable options to replace the M61 in the Phalanx.