As written, the SRFI appears to suggest that a Scheme
is running either wholly in safe or unsafe mode, and
there's no switching between modes.

One might want time-critical code running in unsafe mode,
while the rest of the code runs in safe mode.  Perhaps
the SRFI could suggest syntax for such mode switching,
something like:

 (with-unsafe-arithmetic
    ...
 )

along with the complementary construct for switching
from unsafe to safe mode.

-- Paul