As an experienced user of Keil development package, you can mix assembly and C with no limitation including the optimization that compiler provides. ($REGUSE directive tells the compiler about the register usage in asm - so called register coloring optimization)

!!! But you loose the overview and particulary much more time if you move from C to assembler. So stick to the rule and code as much as possible in C and only some small routines - tasks that are at the edge of 8051 performance in asm (concerning execution speed or code size), or better none if you can do without asm.

Franc