You could try Modula. I have a whole shelf of C compilers, pricey ones
IAR Compass Avocet etc. The basic problem is the 8051 is not designed
for C. go look at http://www.DesignTools.co.nz.
The Modula complier is far tighter, when ever I get the choice I use the
modula. Check out the benchmarks I think they are against the Keil C compiler. They have has real benchmarks for real problems. It is a KCS product but supported out of New Zealand instead of Denmark.