I never realized why Commodore dosen't make a chunky mode for AGA. AGA is born to support more than 5 bitplanes. Simply designers could add a bit in some register to switch to 8-bitplanes and 256 colors chunky mode when set ( like the bit to switch to real 6-8 Bpl mode and EHB-Ham-6/8 mode...).
I think it costs nothing to design the ciruitry in Agnus.
With 8BPL chunky mode, Fetch 4X, and 32 bit access Bandwidth to the ram was theorically 8MB/sec in Lo-Res and and 6.3MB/sec in HiRES, enough to obtains 100FPS in lo-res and 20FPS in hi-res.
What really think is that AGA is an old designed OCS upgrade and no more.
I think Commodore had AGA chipset ready a lot time before launch of A1200 and 4000. I think those machine were forcedly launched too early in response to the growing market of PC and Consoles.
