Actually, there is not really such a thing as "C64 mode".
Unlike the C128, all of the MEGA65 features are available in its C64 configuration. And I mean all of them. Full speed, full RAM access, all VIC-IV features, all 4 SIDs, enhanced opcodes, 32-bit-addressing, everything.
The only difference is that C64 starts in a more developer friendly memory configuration and has the C64 kernal banked in.
Of course you can try remain in C65 configuration and call C65 kernal functions, but keep in mind that it's a hell of a lot more complicated and there's really no benefit, as both C64 and C65 kernal call the same CBDOS routines in the end.
In fact, I suspect, someday the C65 'closed ROM' will vanish into oblivion anyway, as soon as the Open ROM evolves and gets a nice programming language.
(yep, I completely understand why one would want to 'remain' in C65 mode... once I thought so too ;))